Unfortunately, the problem persists even after swapping hardware. I wanted to rule out faulty memory on the machine that crashed all the time. However, using a brandnew ER-8 didn't change anything.
I went back to the most recent non-beta release, which kept on crashing on the original router, too. We'll see what happens on the replacement router.
If that still crashes, I may give the recent beta release a try. If this fails, too, I'll swap all Ubiquiti devices on that location again – using Ciscos.