I see there is no easy way out of it. 
I have been digging through older revisions of the asset and the funny thing is that the “successful” change list (i.e. when I compiled, saved and it was loading correctly from then on) is not that successful, meaning that if I get that revision and recompile - errors start happening again. Somehow I got a successful compile and save once and persisted for some time, but the issue has always been there. I will be posting updates as I go along trying to solve this, maybe it will be useful to someone.
UPDATE 1:
- Successful compile/save was done with 4.10.1. I cannot go back to it, opening that revision with 4.10.4 and recompiling - breaks things.
- 4.12.3, as mentioned crashes on editor load.
UPDATE 2: - I went back to 4.10.4 and deleted the GetActionBP
Class function altogether. SpawnActor from class uses statically ActionGeneric class. This compiles and saves and reloads correctly, including in 4.12.3
UPDATE 3: - After the previous change, if I add “get class defaults”, the one that gives me those 2 arrays of structs, just add the node, without connecting it to anything, save and reload editor - it crashes on 4.12.3. Need to go back to 4.10.4, open and remove it.
UPDATE 4: - I have tried adding arrays of structs locally to the ObjectActions component, so it would not need to query class defaults. Guess what: just declaring those arrays, i.e. creating variables, not using them anywhere, saving, reloading - the issue is back. Actually, it does not even need to be an array, single variable of type struct is enough.