Hi!
Thanks for taking your time to report this. I’m really sorry for this inconvenience.
Based on what you are describing it seems like you need to Flush your permalinks, on the admin of your site.
Also on the same article (shared above), there might be other advices in how to deal with 404 problems. Another thing to double check as well is make sure if there’s any cache plugin active to remove all the caches.
If all of the above does not work, try to install the Transient Manager plugin then go to:
Tools > Transients
And click on Delete Expired Transients, and on Delete Transients with an Expiration.
Please let me know if you have any other questions about this issue or if there’s anything else I can do for you.
Thanks,
Crisoforo