Some plugins from Mantis cannot be installed without first installing the jQuery plugin.
The Mantis-plugin control-panel says that there's a missing dependency: jQuery 1.8.
No matter where I put the jQuery .js file from query.com/download/, the message "missing dependency: jQuery Library 1.8" will not go away, and I cannot install the plugin.
The answer is documented here - but Google has trouble finding it.
Essentially, you have to install the Mantis jQuery plugin - which is not the same as what everybody else calls "jQuery ".
It can be downloaded at github.com/mantisbt-plugins/jquery and gets put in the Mantis' plugin folder.
After copying it and then installing it from the Mantis-plugin control-panel, the other plugin can be installed.
I hope this saves somebody time, it's the result of almost 2 hours of frustration.