If you want
both FR24 and RB24 to work, then you MUST change
(1) receiver="dvbt" to receiver ="avr-tcp"
AND
(2) START_DUMP1090="no" to START_DUMP1090="yes"
(3) must SAVE both files after making above changes
(4) REBOOT Pi
Please do it now even if FR24 stops. We will fix FR24 finally.
Using device 0: Generic RTL2832U OEM
usb_claim_interface error -6
Failed to open rtlsdr device #0.
Above shows that when you issued command "rtl_test -t", either dump 1090-mutability was using the dongle, or RB24's internal dump1090 or may be dump978-rb is using it if you installed dump978-rb by mistake.
Please post output of following command:
apt-cache policy dump978-rb
..