A plugin file with something like this:
filesystem_scm:experimental # comment at the end
Causes the tool to try to download from the following URL:
https://updates.jenkins.io/download/plugins/filesystem_scm-plugin/experimental#commentattheend/filesystem_scm-plugin.hpi
You can see the #commentattheend that gets included in the URL.
Fixed in https://github.com/jenkinsci/plugin-installation-manager-tool/pull/43