We are also getting this problem more frequently. Oddly, it's gotten worse since we installed the 4020 update a couple of weeks ago. I would like to start blanket migration of AC17 projects to AC18, but at the moment I'm going to have to hold off, as AC18 is just not as stable on the few projects we've migrated.
One saving grace... typically, if you choose to work in "Offline" mode, you can continue to work, and a bit later change the status at the top of the Teamwork Palette (click the arrow and select the "Work Offline" option, which has a checkmark, thereby un-checking it). Most of the time it simply returns to Online mode. Sometimes, however, it keeps popping Offline.
We have added several laptops to the network, which seem to be the most prone to this problem, but I'm not certain if it's significant.
We have an extremely robust LAN (Gigabit Ethernet, mostly Macs over AFP, fiber switches, etc.), with full time IT support, and our network appears to be working continuously. We have 2 physical BIM Servers, each running both AC17 and AC18 Modules.
I'm not sure if it's related, but I've also noticed that the BIM Server Manager is not accurately logging Project and User Activity. It will only report from a couple of people. That has me worried about this new BIM Server architecture.
Chuck Kottka
Orcutt Winslow
Phoenix, Arizona, USA
ArchiCAD 25 (since 4.5)
Macbook Pro 15" Touchbar OSX 10.15 Core i7 2.9GHz/16GB RAM/Radeon Pro560 4GB