Hi Leo,
same problem here. To reproduce the issue, I installed a fresh WP, added only GeneratePress & GP Premium and enabled all modules. Then, I imported the test data from WordPress (https://codex.wordpress.org/Theme_Unit_Test) and configured GP to show the primary menu at left sidebar. In that way, I can reproduce the error. I can send you the link to the demo page (but I don’t want to publish the link here). Can you please tell me, how I might send you the link?
Thanks in advance and best regards
Sebastian