[Plugin: WordPress Guest Post Plugin] Can't view or approve posts

Tried with plugin version 2.6, captcha worked.
I created a guest post. I was notified about the post (umlauts in the email are not displayed correctly), but when I clicked the review/approve link, I got a message saying I don’t have sufficient rights to do it, although I’m an administrator.
I see the guest post listed in wp-admin, but when I click approve, I get the same error message again. I also can’t see the contents of the post in wp-admin.

https://www.ads-software.com/extend/plugins/wordpress-guest-post/