I can confirm now with certainty that the source of my random reboots was the Facebook app. It appears to cause increased stress and overheating triggered by unknown address or undefined instruction loops, hence only indirectly connected to 4G or any other activity that uses hardware more intensively.
For info here is my setup:
Fairphone 2 of August 2016
Open FP OS version 18.02.0 with microG version 0.2.4-108-g464d45d, Xposed version 89-sdk23-arm, and GSM location service
Tested after a factory reset and reinstallation of Open FP.
I collected 5 last_kmsg.log files taken with SysLog immediately after random reboots during a 14-hour period. Among those 5 log files I found that facebook.katana appeared as triggering undefined instructions in 3 of them. One was corrupted and can’t be read. One implicated another app but maybe as a result of previous Facebook activity, I’m keeping the other app under observation but for know it doesn’t appear to cause any problem. Uninstalling the Facebook app definitively solves the random reboot problem. It also reduces overheating and improves battery life.
Here a sample of how facebook.katana appears in the crash logs:
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
facebook.katana (15848): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Norm_Shared1 (16098): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Norm_Shared1 (16098): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
audit: audit_lost=21462 audit_rate_limit=20 audit_backlog_limit=64
audit: rate limit exceeded
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared2 (16114): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
Bg_Shared3 (16115): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
Code: fc60 002b 3c5e 002c (defa) 41f0
BgHandler (16077): undefined instruction: pc=b47b22bc
No errors detected