I tested it with new clean Unity Projects in 2021.1.15f1 and 2021.2.0b4, and as soon as I add an Entity package the IL2CPP debug-build will fail, or the release-build will crash on Android.
I guess this is one of the outcome of Notice on DOTS compatibility with Unity 2021.1 .
Because with Unity 2020.3.14f1 the build works.
The only problem I have with this is, that UI Toolkit only works with Dots in Unity 2021.
So I guess I finally ended up in a deadlock 
Though @Endlesser provided a nice work-around for the UI Toolkit problem, maybe that helps. I will give it a try the next days.
Update: The workaround did not work for me, as soon as I have UI Toolkit in the project it fails again in Unity 2020.3.14f1 with the OutOfMemoryError (and all of this in a new clean project)