Thanks for the idea. I did this by renaming my .config/geany folder, then running geany via shortcut. It created a fresh profile, and once I set autocomplete, there was no problem.
I then experimented in the original profile with a fresh filetypes.common file, but it made no difference. Then I removed the geany.conf file, forcing a new one, and the problem was solved.
So I think the geany.conf was corrupted somehow. I'll keep the new one and redo my settings. That should solve the problem I think. If so I'll close this thread.
Thanks again!