dmesg does not work
 

I have a RPi 3B+ connected with a 4 port USB HUB with adapter.
Yesterday after restart of the Pi i had trouble finding ttyUSB0 and ttyUSB1 that where connected directly to the Pi.
Connected to Pi:
16GB USBstick (mounted) USBbackup
4 port USB HUB
mbus to serial converter for Kamstrup 602 heatmeter
RFXcomXL
After restart MBus converter and RFXcom where disconnected. I now connected them to the external USB HUB.
dmesg at first could not find ttyUSB's After another restart it did and i could fix the problem. Now when i typ any dmesg command in conmmandline (Putty) nothing shows however mbus and RFX work in domoticz.
How can i fix this dmesg not showing anything?

Just guessing, your SD card may be dying.

I don't think so because I have been using an eMMC module instead of SD since mid-December.

OK, there must be other logs. I have no Pi running right now, so I cannot check where the kernel log goes, but it must be there somewhere.

Okay, I can see what i can find out.

Found a file named kern.log in /var/log.
its a log list and mostly looks like this:
However i found reboot info at Feb 4 21:51:10. Thats where trouble started.
Wat should i look for in the file?

Any block device or filesystem errors. Maybe running fsck is in order, perhaps there was some filesystem corruption. Just guessing.

I'm going to investigate further tomorrow. Now sleep first.

Sometimes life can be simple if you don't overlook the most obvious solution. Once in a while I swap the eMMC for an SD with an image of the entire system. I did that today. After replacing the eMMC module and restarting it, all USB ports are found again with dmesg | grep -i usb. Solved!:)