**List of steps to reproduce** (step by step, including full links if applicable):
* Edit an article/section (such as https://en.wikipedia.org/w/index.php?title=Oxford%E2%80%93AstraZeneca_COVID-19_vaccine&action=edit§ion=1)
* Click "Show Preview"
*
**What happens?**:
Got this error without the preview:
An error occurred while attempting to preview your changes.
Unrecognized value for parameter "useskin": modern.
**What should have happened instead?**:
Shows HTML preview.
**Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc**:
Windows 7
Firefox 90.0.2 (64-bit)
* This probably has been happening for me in the last week or so.
* I am not using visual editor.
** Skin: Modern
** Preferences:Setting:Show preview without reloading the page
*** Preview with page submission, ie. without this option, is OK.
{F34571755}