Network Sharing Dies Constantly
We are having the same issues since the last Server update, which deprecated file sharing, and relegates it to OS X (we have a mini). Besides the issues you are experiencing, it seems like we went back years in compatibility since High Sierra. 

All of our workstations are Windows 10. When we updated to 10.13, the first issue was anytime opening an AutoCAD file, all MS Excel links required us to acknowledge read-only, which hasn't been an issue in at least a couple years (but it's back). Then when we went to 10.13.2 and the corresponding server app, we had all the problems you mentioned. Additionally, our Windows workstations are slowing to a crawl. Issuing a save command from any application takes at least 5 sec in any application. Windows explorer (desktop and file explorer) crashes when it needs to copy/overwrite a file from the workstation to the server. It actually seems like it's getting worse each day.

Anyway, we've been doing this dance with Apple for 10 years and I don't think they are going to do anything. We just ordered a Synology DS918+ NAS to take over file sharing. I'll keep the mac mini server running Website and Mail until they are deprecated in the next OS X Server "update".