Baffled by EVERY time you open a Microsoft Access Database file, receiving the message "Can't change the default directory to..>" or are you having general Windows errors 1606 such as when you try to install programs or updates?
It's probably due to bad links in the users' Shell folders. Especially if you have recently moved resources on the network, such as upgrading or moving servers. A bad or non-existant path within the Users Shell Folders will cause problems, but fortunately they are easy to check and to correct.
This Microsoft article has all the important information.
Hope this helps someone.