Well, I tried to re-build the binaries from scratch in case something wrong happened during the last build and put them here
http://download.geany.org/snapshots/
The dmg with the _nosign extension contains Geany without any signature at all - it will probably work for you but to run it you'll have to right-click the .app and select Open after which it will permit you to run it.
It would be interesting to know if the new signed binary works or if you still get the same problem.
No other ideas really. There seem to have been many changes related to binary signatures in the last OS X releases, at least judging from
https://developer.apple.com/library/content/technotes/tn2206/_index.html#//a...
So each version makes slightly different checks.