Great!
i did the volatile, the >= and the IRAM_ATTR.
My stacktraces are happening less often.
I don't see it when normally doing HTTP form POST's.
i got an exception after many quick re-POSTs storing the configuration.
it then showed a proper exception format. (looks like i'm out of memory, i guess, bumping my head against the stack?) e.g.
Stack smashing detected.
Exception (5):
epc1=0x4010022f epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000
>>>stack>>>
ctx: sys
sp: 3fffffa0 end: 3fffffb0 offset: 0000
3fffffa0: 3fffdad0 00000000 3fff121c 4010022f
<<<stack<<<
Only during HTTP POST file uploads, it still shows this weird overlapping stacktrace when it fails.
The "config page sometimes printing less than 20 rows" problem, does sometimes still happen.(second last paragraph in my initial post)
ps. i forgot to mention, i disabled the timer altogether a while ago, to see if it would improve stability(so nothing showing on leds). But e.g., it didn't change the problem of the overlapping-exception during receiving a file via HTTP POST.