The error 500 "Token used before issued" when replying to to reviews (see How to Reply to Reviews?) is usually caused by wrong time settings on a web server. If the clocks on your web server are ahead or behind of the correct time (even just for 5 seconds), posting a copy of your reply to cusrev.com might fail with this error.
You can check what is the current time on your web server by going to Settings > General in WordPress admin area. The time is updated every time you re-load the page. You can check if the time shown there is correct. If the clocks are behind or ahead even for several seconds, it can lead to the error.
It is easy to fix this error by correcting the clocks on your web server. If the web server is managed by your hosting company, please reach out to their support team and ask them to correct the time. Most operating systems used on web servers have a feature to automatically set the correct time based on the global time servers. The hosting support should know how to correct the time.
It is important to note that you can use any timezone that you like as long as the time is correct in this timezone.
After the time settings have been corrected, replies to reviews should start working.