Attending button stuck on processing

First of all, love the plugin, thanks for the awesome work!

Since a while (no idea how long the problem existed, a user reported it) the button to attend an event get stuck at processing and then throws an error text.
The same is true when you try to un-attend an event.

The click is registered and the user is attending the event (so the database is updated), the problem lies in the dynamically updating of the button.

The page can only be accessed with valid login so you can use this one for testing:
user: hans_pp
pass: Testpass01

The website is running worpdress v5.3.2 and events manager v5.9.7.2

Hope you can help me out. Thanks in advance!

The page I need help with: [log in to see the link]