Got it! It showed also up in the admin UI already. I have no idea why I did not see it yesterday morning 
Error: “+” and “-” must be surrounded by whitespace in calculations. ╷ 1097 │ width: calc(100%- #{$grid_columns_2_width}); │ ^ ╵ /var/www/discourse/common.scss 1097:19 root stylesheet
Interestingly, this did not pose an issue with the previous version. So something seems to have changed to make scss parsing more strict?
After adding the required whitespace, everything looks great again.
Thank you so much.