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Very interesting issue! I have submitted a patch for this on github.
The attached image code in newbb only worked properly when newbb was the current module (the global $xoopsModule.)
When using the basic system user lookup, there is no current module, and coincidentally, the code causing the fatal error was skipped because the file extensions were not present in a configuration of valid extensions.
When the profile module was used, there was an active module, but the file that defines newbb_attachmentImage() was not loaded because the wrong module directory was being used -- the profile directory, not newbb.
The fix is very simple, in newbb/class/post.php change line 208 from:
include_once $GLOBALS['xoops']->path('modules/' . $xoopsModule->getVar('dirname', 'n') . '/include/functions.image.php');
to:
include_once dirname(__DIR__) . '/include/functions.image.php';
The big mystery was trying to figure out why it ever worked without error ;)
Thanks for the report!