This was an unusual problem we faced in one of our digital products.
While going to the Taxonomy Terms page and changing the url, this error popped up.
Solution
1. Check your .htaccess or WAF rules
2. If there are no rules limiting access to the taxonomy page, go to the URL Alias section
/admin/config/search/path
3. Enter the taxonomy url
/taxonomy/term/your_number
4. You will see a duplicate System path like this
/taxonomy/term/your_number /taxonomy/term/
5. Delete the entry
6. Change the URL
7. Let us say the taxonomy term name is /internet-marketing and the term number is 100, check again by searching in the URL Alias section. You should see something like:
/internet-marketing /taxonomy/term/100
Note 1: The /taxonomy/term/% is the view parameter to the default taxonomy term page. The alias are automatically created with the appropriate number like /taxonomy/term/100
The generic system path - /taxonomy/term url is not required.
Note 2: As always with any advice from the internet, test on a local copy before making system wide changes on a live site