It's possible that one of the 6.0.0 patches became corrupted during download or became corrupted during installation. Fortunately, somewhat, you can manually reinstall both patches. Go here:
http://forum.us.runesofmagic.com/showthread.php?t=31461
Read what Tigsman has to say about downloading and installing the patches, then install them in
ascending order--begin with the patch starting with 5.0.10_2634. Install both patches, then start the game launcher and test to see if the game locks up when switching characters. If it doesn't, you're probably good to go.
If manually re-patching doesn't solve the problem, you will almost certainly need to reinstall the game. Unfortunately, the latest installation files are from mid-Chapter V (5.0.5). You can download these files here:
http://forum.us.runesofmagic.com/showthread.php?t=70835 It doesn't make any difference which set of lniks found in that thread that you use, just so long as you download each of the listed files.
