I can't start BOINC on UBUNTU.
I've got this error message:
GUI RPC bind to port 31416 failed: 98

I can't find any useable solution on the net.

Does anyone have experience with this?

Something is already using TCP port 31416 on your computer, the '98' in the error message tells you that. So you can try to figure out which program uses the port, doing:

lsof -i:31416

from a terminal window.

You can try to exit the BOINC client binary, and restart it, then immediately start BOINC Manager.

Or you can start the BOINC binary and BOINC Manager with different port numbers.
Do:

boinc --gui_rpc_port 31417
boincmgr /g --gui_rpc_port=31417

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