mune: I suggest you try the following:
1) Close Evolution (very important!)
2) Open a terminal (Ctrl-Alt-t), then paste in the following code and hit enter.
3) Restart Evolution
mv ~/.local/share/evolution/mail/local/folders.db ~/.local/share/evolution/mail/local/folders.db_old
4) Wait a short while for Evolution to reindex files.
5) Try to Expunge the Trash
What this exercise does is to rename the Evolution data file "folders.db" to "folders.db_old". As Evolution will not find this file when it starts up, it will automatically generate a new "folders.db" file. If the original "folders.db" was corrupted then this should create a "clean" file.
If there are any problems with this then you can always go back to the original configuration, with the original "folders.db" file, simply by reversing the code in point 2) above.