Hello.
The issue happens without loading staging and out-of-tree modules too (in my case the only non-standard modules are compat-wireless for intel wifi). Moreover, the first time this bug occured I hadn't yet installed compat-wireless (or any other modules).
Unfortunately, I didn't save the log of the error that time, though the problem was ext4-related too, there too was function of kind "ext4...write" in Call trace.
As of non-rt kernel - that bug didn't occur in rt kernel when I was using it. Though I'll try to do some additional testing with standard kernel and report about the results.
Thanks for your time.