@Loeffel - What we do have for a fact is a repeatable scenario with Vladcr that I can replicate on my own machine, and it correlates with information that both I and subsequently Kovid has dug up with reported issues with Python and how it works on Windows. I stand by my comments above in that had your AppData path had accented characters in it, you would not have had any success on Windows in getting plugins using ZipImporter to work.
I had hoped that an "invalid" source directory would also explain your issue as well - from the explanation you have given it would appear that is not the case. As you now have a working system we have no way of repeating the scenario you found yourself in. There are too many variables involved in terms of the choice of plugins and the various changes you will have made to your machine in your attempts to get something working to speculate now about what else might have happened. For instance can you say with 100% certainty that it was exactly the same error message each time you switched Calibre versions? As it could well have been version compatibility issues with the plugins preventing them from installing etc, particularly if you went back to 0.7.46 with a more recent plugin...
If someone else reports the same error, with only ascii characters in any of their paths then of course we will have to try to find an additional reason. Hopefully Kovid's new import mechanism will make this entire issue historical, but only time will tell.