I have had similar issue with a WordPress Multi-site installation. De-activating or re-installing the plugin did not help. So, I have used the following approach.
Connected to the database via PHPmyAdmin
Checked working database table from 1st site in Network of Sites
The name was wp_omgf_fonts
Duplicated this table as wp_5_omgf_fonts
There was another table with the name like wp_omgf_fonts_subsets
Duplicated that table as wp_5_omgf_fonts_subsets.
In my case, both of the tables were empty, but the plugin started to function as expected after this change.