Thank you for helping Zygimantas,
i copied the logfile to another pc.
I was searching now for a way to be able to set settings again, as my guess was that the full flash prevents this, i need a way to delete the big logfile.
Found this post which solved the problem:
I cleared flash by removing: /overlay/upper/usr/var/log/messages in RutOS
then i did a reboot and could disable "save troubleshoot file in flash" (eg in WebGUI) afterwards.
Setting settings works now finally.
Seems like RUT240 has a problem with deleting old logfiles when new ones are available, when saving them in flash.
And also with full flash generally.
Was on FW: RUT2XX_R_00.01.14.5.