Hello,
Thank you for your query and we are so sorry about the trouble this must have caused.
The first variation without the category base is currently accessible on your website and that would indicate that you still have the option to remove the category base enabled.
Please make sure that after making the changes to the category base settings you flush the permalinks by heading over to WordPress Dashboard > Settings > Permalinks and hit the “Save” button once.
Hope this helps solve your issues.
Don’t hesitate to get in touch if you have any other questions.
Made no difference – categories views do not work even trying:
Please make sure that after making the changes to the category base settings you flush the permalinks by heading over to WordPress Dashboard > Settings > Permalinks and hit the “Save” button once.
Hello,
In this case, we might need to take a closer look at the settings. Please edit the first post on this ticket and include your WordPress & FTP logins in the designated Sensitive Data section.
Please do take a complete backup of your website before sharing the information with us.

It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead:
https://wordpress.org/plugins/temporary-login-without-password/
You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any):
https://wordpress.org/plugins/wp-security-audit-log/
Looking forward to helping you.
Thank you.
Hello,
I have updated the sensitive data as requested. Can you please check further?
Thank you.
Hello,
You have a redirection rule that is redirecting the URLs with the category base to the non-category base version:

I deactivated it and now the category URLs are accessible with the base.
Please check and let us know if you need any other assistance.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.