For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
https://yourehome.co.za/index.php?option=com_fabrik&task=plugin.pluginAjax&plugin=digsig&method=ajax_signature_to_image&format=raw&element_id=3397&formid=138&rowid=10&repeatcount=0
<img src="https://yourehome.co.za/index.php?option=com_fabrik&task=plugin.pluginAjax&plugin=digsig&method=ajax_signature_to_image&format=raw&element_id=3397&formid=138&rowid=10&repeatcount=0" />
Tadaaaa! Well, good to hear.
And {rowid}, which you will certainly need), is working for you in the "hardcoded" img tag, but not element placeholders like {tablename___elementname}?
Anyway, you may want to check if you have some ACL set for any other Fabrik stuff (other plugins?), restricting it too much for your use case. If (element) placeholders are working for @troester, they should be working for you as well...