Show inspectors and Previewtheme empty page

Hello,

Recently i'm running Plone 5.1.2 from a Docker Image with Relstorage to use PostgreSQL as persistence layer. Everything is working fine so far. Now I wanted to modify my theme TTW (I just copied from the Barceloneta theme at the beginning).
But "Show inspectors" only creates gray frames and "Preview theme" opens a new blank page. Also on the Barceloneta theme.

I can confirm that the software installation is fine, when I start with a fresh database, the two issues do not exist. Probably at some point I misconfigured something that changed the database. I hope somebody faced the same issue and can point me to the right direction.

Kind regards,
Philip Sahli

I was tracking down the issue in the last days and finally my conclusion is, that when my homepage on / is set to mosaic layout it does only show a blank page after opening previewtheme mode.

https://example.net/++theme++mytheme/@@theming-controlpanel-mapper-getframe?path=/&theme=apply&forms=disable&links=replace&title=Preview:+mytheme -> empty

Changing the URL in querystring field path to a non-Mosaic-Layout page works then.

For the moment I am happy with my workaround. If anybody can reproduce the problem on his site I'm interested to know. Then it would open an issue on the plone.app.mosaic project.

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