X9 stalling and crashing

X9 like its brother x8 is stalling and crashing. I’m now using Catalina, but the same happened in Mojave. Very disappointing. I suspect it’s a memory issue. Restarting the app (X9) makes it run faster and snappier until a certain point, when the problems begin.

The slowness is noticeable inside the app. But the stalling happens when starting an action from within Word. Right now I see an error -1712 after trying to insert a reference into a document in a footnote. 

Crashing happens for no obvious reason. X9 c