It appears that something in the buildbot configuration (typo?) has changed which caused an entire new set of directories for the builder to be created:

@clang-ubsan:/var/lib/buildbot/clang-ubsan$ ls -al
total 68056
drwxr-xr-x 14 buildbot buildbot     4096 Sep 19 14:33 .
drwxr-xr-x  5 buildbot buildbot     4096 Sep 24 02:36 ..
drwx------  3 buildbot buildbot     4096 May 20  2018 2.7.gps-clang-ubsan
drwx------  3 buildbot buildbot     4096 Sep 24 02:43 2.7.gps-clang-ubsan.clang-usban
drwx------  3 buildbot buildbot     4096 May 20  2018 3.6.gps-clang-ubsan
drwx------  3 buildbot buildbot     4096 May 20  2018 3.7.gps-clang-ubsan
drwx------  3 buildbot buildbot     4096 Sep 19 16:41 3.7.gps-clang-ubsan.clang-usban
drwx------  3 buildbot buildbot     4096 Jun  4 20:30 3.8.gps-clang-ubsan
drwx------  3 buildbot buildbot     4096 Sep 19 16:05 3.8.gps-clang-ubsan.clang-usban
drwx------  3 buildbot buildbot     4096 May 20  2018 3.x.gps-clang-ubsan
drwx------  3 buildbot buildbot     4096 Sep 19 14:39 3.x.gps-clang-ubsan.clang-usban
-rw-------  1 buildbot buildbot     1333 May 20  2018 buildbot.tac
drwx------  3 buildbot buildbot     4096 Jun  2  2018 custom.gps-clang-ubsan
drwx------  2 buildbot buildbot     4096 Sep 19 14:33 custom.gps-clang-ubsan.clang-usban
drwxr-xr-x  2 buildbot buildbot     4096 May 20  2018 info
-rw-------  1 buildbot buildbot       12 May 15 22:14 twistd.hostname
-rw-r--r--  1 buildbot buildbot  9574124 Sep 25 20:39 twistd.log
-rw-r--r--  1 buildbot buildbot 10000179 Jul 30 18:35 twistd.log.1
-rw-r--r--  1 buildbot buildbot 10000101 Jun  2 22:17 twistd.log.2
-rw-r--r--  1 buildbot buildbot 10000025 Mar  9  2019 twistd.log.3
-rw-r--r--  1 buildbot buildbot 10000056 Dec 10  2018 twistd.log.4
-rw-r--r--  1 buildbot buildbot 10000014 Oct  9  2018 twistd.log.5
-rw-r--r--  1 buildbot buildbot 10000168 Jul 24  2018 twistd.log.6
-rw-------  1 buildbot buildbot        3 May 15 22:14 twistd.pid


Notice the directories named -clang-ubsan.clang-usban that appeared on September 19th.  From twistd.log:

2019-09-19 14:33:20+0000 [Broker,client] Lost connection to buildbot.python.org:9020
2019-09-19 14:33:20+0000 [Broker,client] <twisted.internet.tcp.Connector instance at 0x7fae10e69c68> will retry in 3 seconds
2019-09-19 14:33:20+0000 [-] Stopping factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:23+0000 [-] Starting factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:23+0000 [-] Connecting to buildbot.python.org:9020
2019-09-19 14:33:23+0000 [Uninitialized] Connection to buildbot.python.org:9020 failed: Connection Refused
2019-09-19 14:33:23+0000 [Uninitialized] <twisted.internet.tcp.Connector instance at 0x7fae10e69c68> will retry in 8 seconds
2019-09-19 14:33:23+0000 [-] Stopping factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:32+0000 [-] Starting factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:32+0000 [-] Connecting to buildbot.python.org:9020
2019-09-19 14:33:32+0000 [Uninitialized] Connection to buildbot.python.org:9020 failed: Connection Refused
2019-09-19 14:33:32+0000 [Uninitialized] <twisted.internet.tcp.Connector instance at 0x7fae10e69c68> will retry in 22 seconds
2019-09-19 14:33:32+0000 [-] Stopping factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:54+0000 [-] Starting factory <buildslave.bot.BotFactory instance at 0x7fae10e697a0>
2019-09-19 14:33:54+0000 [-] Connecting to buildbot.python.org:9020
2019-09-19 14:33:55+0000 [Broker,client] message from master: attached
2019-09-19 14:33:55+0000 [Broker,client] Peer will receive following PB traceback:
2019-09-19 14:33:55+0000 [Broker,client] Unhandled Error
        Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/spread/banana.py", line 173, in gotItem
            self.callExpressionReceived(item)
          File "/usr/lib/python2.7/dist-packages/twisted/spread/banana.py", line 136, in callExpressionReceived
            self.expressionReceived(obj)
          File "/usr/lib/python2.7/dist-packages/twisted/spread/pb.py", line 575, in expressionReceived
            method(*sexp[1:])
          File "/usr/lib/python2.7/dist-packages/twisted/spread/pb.py", line 896, in proto_message
            self._recvMessage(self.localObjectForID, requestID, objectID, message, answerRequired, netArgs, netKw)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/spread/pb.py", line 913, in _recvMessage
            netResult = object.remoteMessageReceived(self, message, netArgs, netKw)
          File "/usr/lib/python2.7/dist-packages/twisted/spread/flavors.py", line 118, in remoteMessageReceived
            raise NoSuchMethod("No such method: remote_%s" % (message,))
        twisted.spread.flavors.NoSuchMethod: No such method: remote_getWorkerInfo
        
2019-09-19 14:33:55+0000 [Broker,client] message from master: buildbot-slave detected, failing back to deprecated buildslave API. (I
gnoring missing getWorkerInfo method.)
2019-09-19 14:33:55+0000 [Broker,client] changing builddir for builder AMD64 Clang UBSan 2.7 from 2.7.gps-clang-ubsan to 2.7.gps-clang-ubsan.clang-usban
2019-09-19 14:33:55+0000 [Broker,client] changing builddir for builder AMD64 Clang UBSan 3.8 from 3.8.gps-clang-ubsan to 3.8.gps-clang-ubsan.clang-usban
2019-09-19 14:33:55+0000 [Broker,client] changing builddir for builder AMD64 Clang UBSan 3.7 from 3.7.gps-clang-ubsan to 3.7.gps-clang-ubsan.clang-usban
2019-09-19 14:33:55+0000 [Broker,client] changing builddir for builder AMD64 Clang UBSan 3.x from 3.x.gps-clang-ubsan to 3.x.gps-clang-ubsan.clang-usban
2019-09-19 14:33:55+0000 [Broker,client] changing builddir for builder AMD64 Clang UBSan custom from custom.gps-clang-ubsan to custom.gps-clang-ubsan.clang-usban
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory '3.7.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory 'custom.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory '3.6.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory '3.x.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory '3.8.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:55+0000 [Broker,client] I have a leftover directory '2.7.gps-clang-ubsan' that is not being used by the buildmaster: you can delete it now
2019-09-19 14:33:56+0000 [Broker,client] message from master: attached
2019-09-19 14:33:56+0000 [Broker,client] message from master: attached
2019-09-19 14:33:56+0000 [Broker,client] message from master: attached
2019-09-19 14:33:56+0000 [Broker,client] message from master: attached
2019-09-19 14:33:56+0000 [Broker,client] message from master: attached
2019-09-19 14:33:56+0000 [Broker,client] Connected to buildbot.python.org:9020; slave is ready
2019-09-19 14:33:56+0000 [Broker,client] sending application-level keepalives every 600 seconds
2019-09-19 14:39:12+0000 [Broker,client] message from master: ping