The add-on for ArchiCAD 8.0 aren't AC 8.1 compatible.
I suppose that you have an outdated add-on (that AC 8.1 doesn't load) and you are opening one project where you used that add-on.
Try to find out which are the not-standard add-ons you have in your ArchiCAD folder and remove them then start ArchiCAD.
Another issue (I already addressed it to GS) is that if you manually load an add-on from another location than the standard one (add-on folder) and then you delete/move that add-on from that location the related Register entry of ArchiCAD will still conserve that location, trying to access to that add-on each time you start ArchiCAD.
Fabrizio Diodati Graphisoft Italy Srl | Via Rossignago 2/A Spinea Venezia 30038 Italy
If you are working on a project upgraded from 8.0, it is possible an add-on used in the project is not loaded in 8.1. Try using the Add-on Manager (Tools) from within the project to see what was loaded in 8.0 or what is missing in 8.1. Perhaps you can uncheck/unload the culprit from there. (I have not had this problem, so I can't be sure missing add-ons show up here.) You might need to open the project in both 8 & 8.1 to compare loaded Add-ons.
The ArchiCAD 8 & 8.1 folders contain add-ons in folders called 'Add-Ons' and 'Goodies'. If you find what's missing in the 'Goodies' folder, use the Add-on Manager to load it if you need it for the project.
Mabe Ng Roche
Cutler Anderson Architects
Bainbridge Island, WA USA
G5 dual/1 mb/128mb ATI Radeon/(2) 19" Sony lcds