CSS optimization error creates white screen

Problem/Motivation

After the last upgrade which took me to Drupal 10.3.5, I got a WSOD (white screen). I report here because it seems to be related to using the theme, if I change to the Claro admin theme there is no white screen. I have cleared the cache repeatedly.

The error I see is:

Uncaught PHP Exception Exception: "Only file CSS assets can be optimized." at ../web/core/lib/Drupal/Core/Asset/CssOptimizer.php line 43

Steps to reproduce

Awesome Symphonies Theme 3.0.1 under Drupal 10.3.5, white screen.

Proposed resolution

If I turned off CSS optimization with the Development module the issue dissapears.