Contao 2.10.3 bug? Multiple sites and layout error message

This weekend I attempted to manually upgrade my client's installation from Typolight 2.7.5 to Contao 2.10.3.

I experience two major issues; The first was that upgrading the database directly to 2.10.3 did not work (covered in my reply to another thread), and the second was that I received error messages related to layouts.

My client is running a single Contao installation, serving about 15 different web sites/domains. Now, the "main" site worked fine, but all other sites returned a "No layout found" error message. I checked Contao for problems with the page layouts or front end modules for the different sites, but could find no errors. All sites use the same page template, by the way.

The system log only stated something like "Layout with ID 7 could not be found".

This issue is not present in 2.9.5. Rolling back to Typolight 2.7.5 and only upgrading to Contao 2.9.5 fixed the problem.