The cause for this error is any cause for not being able to load a module. This includes version conflicts, so when trying to load a virtualenv with python 3.3 interpreter by a uWSGI embedded python3.4 interpreter the action fails.

This is - bluntly overstated - a design flaw or overly confident claim with respect to virtualenv support in uWSGI. This configuration can never work and becomes a configuration error.

It really deserves a less generic error message, indicating the version conflict and possibly the path to the encoding module it tried to load. This helps in debugging the cause and possibly abandon the idea before wading through the list of reports generated by this error message (only one of which I found to hint at the version conflict and most don't have a clear solution or different root cause).