Win7 clients can't connect to OSX Server after 10.10.2 and Server 4.0.3 update

I'm troubleshooting an annoying issue with our Yosemite fileserver at work and hoping someone has suggestions.

The server is running Yosemite, clients a mix of MacOS from 10.6 through 10.10, and some Windows 7 boxes.

With the server on 10.10.1 and the just-prior version of Server (4.0, I assume; were 4.0.1 and 4.0.2 ever available to the public?), everything basically worked. Last night, I updated the server to 10.10.2 and 4.0.3. Nothing unusual during the update apart from a bunch of spotlight indexing processes running as individual users sucking up a lot of CPU for the first few minutes, which may be normal.

However, now almost all the Macs are fine, but the Windows clients are all getting permissions errors when trying to connect. Even weirder, one Yosemite client was also getting an error on login when trying to connect via SMB, both on 10.10.1 and 10.10.2; the same computer connecting via AFP works fine.

I'm inclined to think that it has something to do with the switch to SMB3 mentioned in the changelog for Server 4.0.3, although I'm not 100% sure that didn't exist under 4.0 as well, and if this is the problem I'm not sure what to try to fix it.

I'm already aware of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\LmCompatibilityLevel needing to be at least 3 (or non-existent, so it defaults to 3 on Win7) to work with Yosemite; these clients already have that registry key set. Suggestions?