Back in Nov 14-16 2019 I found a way to get
linux boincmgr to find the project (as opposed to asking me to set up at least
one project).

However, there is a simple way to diagnose the problem
if you have strace.

strace boinccmd --set_run_mode never

will let you know what is going wrong.

Here is a working set as an example. The files can contain any
string of bytes as long as they match.

rw-r----- 1 davea davea 7 Feb 26 15:10 /var/lib/boinc/gui_rpc_auth.cfg
0 6d796163 63740a myacct.
lrwxrwxrwx 1 davea davea 31 Feb 28 08:51 /home/davea/gui_rpc_auth.cfg -> /var/lib/boinc/gui_rpc_auth.cfg
0 6d796163 63740a myacct.

The following may not be needed, but cannot hurt
-rw-r--r-- 1 root root 7 Feb 26 15:10 /etc/boinc-client/gui_rpc_auth.cfg
0 6d796163 63740a myacct.
-rw-r----- 1 davea davea 7 Feb 26 15:10 /var/lib/boinc-client/gui_rpc_auth.cfg
0 6d796163 63740a myacct.

Perhaps this will help someone. I hope so. I just had it happen *again*
on just one seti machine after an update.

The previous was just....wrong. I knew this yesterday during
the weekly outage, so now I'll correct the mistakes and show a
shell script any Unix/Linux user might find useful
if and only if one is having trouble connecting to existing projects
with boincmgr or boinccmd.

(Below using xxd, a standard Linux hex dumper program
to display the file content.)
A correct example of the gui rpc cfg files is (remember the
contents of each should match the others but any
content will work), followed by a shell script that
I found useful to create.

-rw-r--r-- 1 root root 7 Feb 26 15:10 /etc/boinc-client/gui_rpc_auth.cfg
00000000: 6d79 6163 6374 0a myacct.

-rw-r----- 1 boinc boinc 7 Feb 26 15:10 /var/lib/boinc/gui_rpc_auth.cfg
00000000: 6d79 6163 6374 0a myacct.

-rw-r----- 1 boinc boinc 7 Feb 26 15:10 /var/lib/boinc-client/gui_rpc_auth.cfg
00000000: 6d79 6163 6374 0a myacct.

owner of of /home/davea/gui_rpc_auth.cfg should be davea
-rw-rw-r-- 1 davea davea 7 Mar 3 11:13 /home/davea/gui_rpc_auth.cfg
00000000: 6d79 6163 6374 0a myacct.

#!/bin/sh
# In case you have a system with boinc project(s) and
# boincmgr and boinccmd cannot see the project(s)
# you may have an authorization error.

# the command
# strace boinccmd --set_run_mode never
# may help to see if it is an authorization issue.

#The following is commented so it just shows what is what.
#To actually do anything to the files uncomment lines
#as necessary.

# Pick a hex dump program you have on hand. xxd is standard Linux.
hx=xxd
auth="myacct"

targ=/etc/boinc-client/gui_rpc_auth.cfg
#echo "$auth" >$targ
ls -l  $targ
$hx $targ
ls -l $targ >/tmp/auth 
grep 'root root' </tmp/auth 1>/dev/null 2>/dev/null
if [ $? -ne 0 ]
then
  echo "ERROR! $targ should be owner and group root"
fi
 
echo ""
targ=/var/lib/boinc/gui_rpc_auth.cfg
#echo "$auth" >$targ
sudo ls -l $targ
sudo $hx $targ
sudo ls -l $targ >/tmp/auth
sudo chown davea /tmp/auth
grep 'boinc boinc' </tmp/auth 1>/dev/null 2>/dev/null
if [ $? -ne 0 ]
then
  echo "ERROR! $targ must be owner and group boinc"
  #sudo chown boinc $targ
  #sudo chgrp boinc $targ
fi

echo ""
targ=/var/lib/boinc-client/gui_rpc_auth.cfg
#echo "$auth" >$targ
sudo ls -l $targ
sudo $hx $targ
sudo ls -l $targ >/tmp/auth
sudo chown davea /tmp/auth
grep 'boinc boinc' </tmp/auth 1>/dev/null 2>/dev/null
if [ $? -ne 0 ]
then
  echo "ERROR! $targ must be owner and group boinc"
  #sudo chown boinc $targ
  #sudo chgrp boinc $targ
fi

echo ""
targ="$HOME/gui_rpc_auth.cfg"
#echo "$auth" >$targ
echo "owner of of $targ should be $USER" 
ls -l $targ
$hx $targ
exit