Been reviewing some older posts about problems like this.
First step is to turn on debugging and make sure there are no PHP errors when submitting the page, better yet, make sure there are no php errors. Any site that is live should run without generating errors and warnings. If you have other plugins or your theme is generating errors or warnings then these should be reported to the authors. https://support.advancedcustomfields.com/forums/topic/cant-save-post-if-i-add-a-url-to-a-texturl-field/
Next step https://www.georgebuckingham.com/blog/wordpress-404-error-with-advanced-custom-fields/
Checking console errors suggested by @alfonsoc is also a very good idea.
If all else fails then disable all other plugins and see if you can get it to go away that way.
There are a lot of topics here about this, but I have never seen the issue. So it’s very hard to debug. If you have a site that’s free of errors and this problem persists on a WP site on the same server with noting but WP, ACF and one of the default 20XX themes installed then the best hope you have is to submit a support ticket https://support.advancedcustomfields.com/new-ticket/
