I really start to hate this Forum Engine ..
However here some workaround for sending Private Messages to "Corrupted" Users:
Instead of clicking on Username click on number of Posts which will lead you to URL like:
http://community.vsl.co.at/search/SearchResults.aspx?u=31740&o=DateDescending
Note down everything between ?u= and &o= (in this case 31740)
Then open the Page http://community.vsl.co.at/forums/AddPost.aspx?ForumID=0&UserId=31740 (where the last digits are the ID extracted in previous step)
Unfortunately I can not test if retreiving is really working, or if these "affected" Users also have issues retreiving PMs
Best regards
Gabriel