I gleaned some info from a guy that used WireShark to track down his problem with error H202.
Apparently if you open the file as \\fileserver.mydomain.com\fileshare\quickbooks.qbw, Quickbooks may have problems reaching the database server to switch to multi-user mode.
However, if you open the file as \\fileserver\fileshare\quickbooks.qbw, Quickbooks should work just fine. i.e. don't use the fully qualified domain name, use the NETBIOS name.
In our case this meant editing the
lmhosts.sam file located at
c:\windows\system32\drivers\etc to include the line
10.1.1.19 FILESERVER #PRE
(where 10.1.1.19 is the ip of the fileserver your QB file is on and FILESERVER is the NETBIOS name of the server)
You can also see that this is your problem from Quickbooks point of view if you specify the full name in the "Change hosting server" field on the diagnostic tool.