Hi,
Thanks for your response! The .pst file was definitely not in use when I
backed it up, but it also wasn't the only file I couldn't decrypt.
In the meantime I was able to figure out what the problem was: The fact that
I was unable to decrypt some of my encrypted files was because I had Symantec
AntiVirus Corporate Edition 10.2 installed (see
http://service1.symantec.com/SUPPORT...07020810462848). After I uninstalled it, all files decrypted again fine.