Please help us help you and fill where relevant:
Your LimeSurvey version: Version 2.72.3+171020
Own server or LimeSurvey hosting: Own
Theme: N/A
==================
Sometimes when trying to save or save and close the email template form we get a 403.  Do you have any ideas what could be causing this or where to look for more info?

1. 1. Request URL:
      ***quality/index.php?r=admin/emailtemplates/sa/update/surveyid/999999
   2. Request Method:
      POST
   3. Status Code:
      403 Forbidden
   4. Remote Address:
      ***
   5. Referrer Policy:
      no-referrer-when-downgrade
2. Response HeadersView source
   1. Cache-Control:
      no-cache
   2. Connection:
      close
   3. Content-Type:
      text/html
3. Request HeadersView source
   1. Accept:
      text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
   2. Accept-Encoding:
      gzip, deflate, br
   3. Accept-Language:
      en-US,en;q=0.9
   4. Cache-Control:
      max-age=0
   5. Connection:
      keep-alive
   6. Content-Length:
      4190
   7. Content-Type:
      application/x-www-form-urlencoded
   8. Cookie:
      LS=***; PHPSESSID=1vfnltotk6pkrnqceubs2f85p6; YII_CSRF_TOKEN=TmpmZHU4MDMyR285a25GZDBnbkhZZGZ6UVV5S3VtTzNzOtUKde5eMcYDvan1YJ-Ic-FLh18on8Ui8HC0K67oxQ%3D%3D; FMLA=FMLA-N2
   9. Host:
      ***
   10. Origin:
       ***
   11. Referer:
       ***/quality/index.php?r=admin/emailtemplates/sa/index/surveyid/999999
   12. sec-ch-ua:
       " Not A;Brand";v="99", "Chromium";v="96", "Google Chrome";v="96"
   13. sec-ch-ua-mobile:
       ?0
   14. sec-ch-ua-platform:
       "Windows"
   15. Sec-Fetch-Dest:
       document
   16. Sec-Fetch-Mode:
       navigate
   17. Sec-Fetch-Site:
       same-origin
   18. Sec-Fetch-User:
       ?1
   19. Upgrade-Insecure-Requests:
       1
   20. User-Agent:
       Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36

Thanks,
Kyle

Hard to tell without checking the server logfiles. If you cannot reproduce the issue, it will be long way. I would start with using e.g Firefox for a longer time and see if the issue is reoccurring.

With a LimeSurvey version from 2017 which is no longer receiving any update, you will get more and more issues.

There are server security system that disable this action and send a 403.

Look at your server log : you have the reason somewhere.