I run Slackware64 boinc-7.18.1-x86_64-2_SBo to (incrementing SlackBuild) boinc-7.20.2-x86_64-1_SBo (work fine), Slackware-arm boinc-7.18.1-arm-2_SBo to boinc-7.20.2-arm-1_SBo (built from SlackBuilds.org (SBo).
When /etc/rc.d/rc.boinc does (in existent/boinc-writable data directory) 'exec /usr/boinc/boinccmd --dir /home/user/.boinc_data --redirectio --join_acct_mgr https://grcpool.com/ user password' I get cryptic/useless error message ': No such file or directory'. What file/directory? (message from BOINC, not the OS: doesn't do ones starting with colon).
    Please write clear/useful error messages but for now is there a configure option to recompile with debugging so I might get a clear/useful error message?
    If debugging would help others, please message me (I don't come here much).
    Please allow people to automatically get notifications of replies to their own threads, and don't wrongly close threads for being 'old'.

Looks like you mix command options from 'boinc' and 'boinccmd'.

Check valid options running:

/usr/boinc/boinc --help
/usr/boinc/boinccmd --help

I made a mistake. The commands being done are these. I maybe found out the problem: it was with using a new rc file (my old one works).

BOINC_DIR=/home/u/.boinc_data    # directory of boinc files
BOINC_BIN=boinc                  # name of the boinc binary
BOINC_USER=u                     # user that will run boinc process
BOINC_OPTIONS="--dir $BOINC_DIR --redirectio"
su - $BOINC_USER -c "cd $BOINC_DIR; exec /usr/bin/BOINC_BIN $BOINC_OPTIONS" &
su - $BOINC_USER -c "cd $BOINC_DIR; exec /usr/bin/boinccmd --join_acct_mgr https://grcpool.com/ user pass"

The old one didn't always work either, but copying & pasting the commands did, so it must've been a different problem with the system ($PATH or console character set).

su - $BOINC_USER -c "cd $BOINC_DIR; exec /usr/bin/BOINC_BIN $BOINC_OPTIONS" &
su - $BOINC_USER -c "cd $BOINC_DIR; exec /usr/bin/boinccmd --join_acct_mgr https://grcpool.com/ user pass"

Sending the first command bg might cause a race condition since BOINC needs a short while to be completely up.
The 2nd command requires a fully running BOINC client.
You may consider to remove the "&" insert a "sleep x" after the 1st command.

[...] Sending the first command bg might cause a race condition since BOINC needs a short while to be completely up. [...]

Thanks. I did also rewrite it to be all on one line (separated by semicolons) but same happened... even if I don't launch the manager, the same happens: something is going wrong with launching boinc itself from a script, regardless whether any manager is even run.

Instead of this
/usr/bin/BOINC_BIN

you may try this
/usr/bin/$BOINC_BIN