This seems to be only affecting groups that allow membership requests, but have not added any text to the group’s “Custom template to display to users” field.
The error using Chrome on Ubuntu is: index.js:930 Uncaught TypeError: Cannot set property reason of #<RequestGroupMembershipForm> which has only a getter
In our case, we deliberately tend to leave that blank to encourage the user to give us an actual personal reason. Otherwise, they just click Submit Request with the template text.
Good to know that there is a workaround by simply adding some text in there.