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 data directory) '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'. Unclear what directory... same hard-coded user-directory build script works fine on x86(64) and used to on arm32 but most recent builds return this error message.
        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).

Your operating system is what gives this error, not BOINC. BOINC hasn't even started yet due to either the directory being missing or the file you try to start.

Looks like you called "boinccmd ..." without a path.
Either
- include the directory where "boinccmd" is located in the path
- call "boinccmd" with a preceding path, e.g. "./boinccmd" or "/absolute/path/to/the/executable/boinccmd".