I experience the same Issue and can confirm it, but on a different System and ArchiCad Version.
System has two Monitors. In addition tried to move/close all floating Windows and Panels to the Primary Screen, without any result.
ArchiCad Version 21 (6013-1.2)
Teamwork Project on ArchiCad Server 22.214.171.12402
Windows 10 Pro (1809)
Core i5-6400 (2.7/ 3.30 GHz)
16GB RAM, PNY PCIe 2GB Quadro K620, Drivers are up to date
Mouse: MS Wireless 1000
Admin of BIM [Cloud] [SaaS] [Server] | AC 19-25 [Win&Mac Clients] | VM's