Thunderbird: Crash on saving attachments

Hi everyone,
I have encountered the following problem with thunderbird: Every time I try to save an attachment (“save as”), thunderbird freezes. By now, I have found out that this is due to the default saving path being set to a virtual network drive that is online available through VPN connection. Is there a way I can change this default path to another location?
[Setting the default path for saving attachments in the preferences did not resolve the problem. I suspect that the solution might be somewhere in the config editor, but haven’t found the corresponding command so far…]

Thanks in advance!