'The connection was reset' on WordPress when creating an activity

Hello.

I am using WordPress 5.4.2 with verison 1.15 of the H5P plugin. When using WordPress I am on Firefox desktop version 77.

Using Firefox and Chrome, I have tried to add a new H5P activity. I can go through the steps to put together an activity, but when I press 'Create' after a while, I eventually get a page saying 'The connection was reset'. I have tried Drag and Drop, Text Drag and Drop and both activities have this problem.

I have tried to create a new page in WordPress which works absolutely fine. I also upped the PHP execution time to 300 using the instructions on this website, but this didn't help either.

Do you have any idea what might be causing this? Please let me know what information you need and I'll do my best to get it.