I have hundreds of computers that are logging into old 5.1 servers
presently that have old versions of Client32 on them and have been logging
in fine for years. All of a sudden we are getting calls that users are
getting Tree are server not found on these pc's.

We upgrade the client to version 4.91 and the problem goes away but not
sure why it has started to begin with.

We have two 6.5 servers in our tree that we have begun to migrate users to
them and we have had "Tree or server not found" on these computers once
migrated and were able to resolve by setting up SLP and then configuring
the Service location tab on them.

What could be causing this on old servers all of a sudden ?