Adding comment not working (6.3.2)

On our server (v6.3.2) in the web GUI adding a comment to a file in the Library does not work.

If I select the Comment menu, the comment field appears on the right, I can enter a comment, but clicking on Submit button does nothing.

I use Firefox v61.0.2

Is this a known issue? Or maybe a configuration issue?

Same here. CE 6.3.2 Ubuntu not working with either Firefox or Chromium.

Still not working in CE 6.3.3.

Error message in Firefox:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

Error in Chromium:

Uncaught SyntaxError: Unexpected token < in JSON at position 0

Don’t know if this is same case, but regarding to this for my raspberry 6.3.2 installation I had to migrate old comments using this command:
./ python-env seahub/ migrate_file_comment
(execute command with seafile running!)

Version 6.3 also changed the database table for file comments, if you have used this feature, you need migrate old file comments using the following commends after upgrading to 6.3: