Checkbox 'push changes immediately' is greyed out and cannot be activated

Since the update to 3.4.8 I cannot activate the checkbox 'push changes immediately to origin/master' anymore. It doesn't matter which branch is checked out. The text shows the correct tracked remote branch that already exists (in GitLab, where I want to push to).

After committing it is possible to push to origin by a simple click on the 'Push' menu button and confirming the following dialog without changing anything.

This issue came with version 3.4.8.