Did the directory name "templates" become a reserved word in 3.7/3.8? There is an atto_plugin called "templates" that the plugin system will not see. No errors, it simply doesn't show up as available to install. Renaming the directory to "template" allows it to be seen by the system, albeit with errors.

This does install in 3.5 and 3.6.

This may be a regression caused by MDL-67768.

See this forum for discussion.