Custom repositories are broken with Maintenancetool 3.0.5

With the recent update of the Maintenancetool backend to version 3.0.5, custom repositories are completly broken.

Before the update, we could add a custom repsoitory in the custom section and enable it by unchecking the "Only use temporary..." checkbox.

With 3.0.5, this is broken. Doing so will show an indeterminated progress "fetching repositories" and then return to the welcome page, with the previously made repository changes reset. This also applies to repositories manually added as temporary. I ran the tool in verbose mode, but nothing was printed on the console for that phase.

Note: I am referring to the official open source Qt online installer which got that update today. So it's theortically possible this is a special situation caused by customizations done in the Qt installer itself. In that case, please move the issue to the according project/component.