I hit this issue with an "H:" homedrive that is on a network share, and then in Windows is using "offline files" to keep a local copy.   .idlerc was not cached so IDLE worked when online/connected to my work network but not when I was offline. 

The temporary workaround was to mark .idlerc as "available offline" when connected.  I wanted to document this in case others hit this scenario.  

I also noticed that there is a .idlerc directory created in my local user directory c:\users\[username\.idlerc.  This one has the recent-files.lst in it.  So IDLE is creating two copies of .idlerc - one in the environment-variable-defined (and roaming) home directory, and one in the default local user directory.   I would suggest that as this bug is investigated, you keep track of both instances of .idlerc.