Ok I fixed my problem, I used another SD card, just copied "Nintendo 3DS" folder from the other one and made clean ARN 5.0 install on it.
Now everything is working as expected...I feel dumb to not have though about this earlier...
I formated the other one and cloned content and it's working too...Still don't understand why it was not working before but, at least it work now, problem solved...the weird way...
At least I was right about my TWL and AGB firms, they were clean. Anyway, thx all for you help and sorry for that.
Glad it's fixed. But now I'm wondering, how a SD could lead to twl & agb firms failure only.
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the titleIDs to redownload via 3DSNUS should be 0004013800000202 (AGB_FIRM) and 0004013800000102 (TWL_Firm) No?
I'll let you look yourself. Check the 2 threads in the tutorials section.