iFolder 2.1.8 -- intermittent "Authentication failed"

We have an iFolder 2.1.8 server running on NetWare 6.5 SP7 that's been
in use since 2003. It is scheduled to be retired but we still have
several VIPs using it. Recently, said VIPs have been getting
intermittent "authentication failed" messages when they try to login.
There's nothing visibly wrong with their user accounts in eDirectory:
account and password are not expired, login is not disabled or locked
out. I haven't tried restarting the iFolder app or the server itself
because I don't know what I'll do if it doesn't come back up -- it
hasn't been rebooted in almost 18 months.

Can anyone think of something to look at?