Projects show up greyed out in File Select dialog for “Place Hotlink…” if they are already opened by another user who uses macOS 10.13 High Sierra.
It is due to Apple's significant changes on how smb works in the background.
Workaround
To prevent this issue use Teamwork files as hotlink modules instead of Solo projects.
Solution
This issue should be fixed in the latest update of ARCHICAD 21 (build 6013), and we plan to include a fix for this issue in the 8000 Update of ARCHICAD 20.