As discussed, it could be that someone else has the DWG open.
I have also found on a Windows system, just having a file selected in Widows Explorer with the 'preview' panel active, is enough for Archicad to think the file is in use.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10