Fixed with the latest release.
It was caused by a special entity in the Tiberium mod, which had 32768 module slots. The game wasn't happy when trying to make the "default modules" gui with that many buttons.
The update now ignores such entities - so the gui is limited to something that can actually be created, but also means the inserter won't work right with those entities (hopefully the limit is still high enough there's no entities you would actually want to interact with that get ignored).
In the future, I might try change the gui somehow to support large module counts instead...