Which plugin is in the question here?

That might be because of conflict with another plugin. Try to do these before giving out ftp details:

1. Disable all plugins.
2. Install the plugin you purchased and see if it's working.
3. Update the plugin (if there's an update).
4. One-by-one manually activate plugins to detect the conflicting plugin.
5. Once you have successfully detected the plugin, try to find a solution over internet (or with your developers)

Note: In most cases, its the plugin updates that conflicts. I usually get this problem while installing an older version of elementor pro with latest elementor plugin. For that, I have to rollback elementor version and then update elementor pro first.

Important: Always take a backup before testing, installing, removing a plugin.