Steps to replicate the issue (include links if applicable):

• access the mobile version of Wikipedia logged in
• go to your personal JavaScript or CSS (Special:Mypage/common.js)
• make a change and click to save the page

What happens?:
A warning appears, stating that the page will load in seconds, but it doesn't. If you click the link to reload the page, the change disappears.

What should have happened instead?:
Previously it was possible to save or verify changes.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
The error occurs on any mobile version page that is edited using "?action=submit"