Upgrade possibilities after expiration

Also check within CBSubs > Settings > Display > Expiration and ensure all the URLs under "Expiration and restricted access redirects" are blank.

Fields are blank.

You need to ensure you've a public and published Joomla menu link to CBs profile page. If it's redirecting you to login then you are missing that link.

Indeed, my Joomla link to CB's profile page was set to registered. For a second I thought that was it, however no luck yet.
Now as soon as I login with an expired account, the link is as follows: www.eagen.org/index.php/my-profile/pluginclass?plugin=cbpaidsubscriptions&cbpaccount=expired&user=998

Perfect, but where the plans should be presented, the line 'Browser Cookies must be enabled' is shown.
I quickly searched the forum first for similar issues ( www.joomlapolis.com/forum/255-developer-members-support/222862-browser-cookies-must-be-enabled-but-they-are#234528 ).
I started with disabling non-core CB plugins. As soon as I disable the CB Paid Subscriptions plugin, 'Browser Cookies must be enabled' is not shown anymore. Instead the content is just blank (edit: As it should since the plugin must be activated, so thats not the problem, sorry! )
When I try to login again; Error Login denied! Your account has either been blocked or you have not activated it yet.

Bad server configuration? Debug mode does not reveal anything (but I am not a very experienced user...).
Thanks...

Disable spoof checking within CB > Configuration > Integrations and see if the cookie issue goes away or check your browser settings to ensure cookies are configured correctly. The landing page (plan expiration) looks correct and should allow for renewal.

Followed your advice, but still not solved.
Either I get the message Please login first (again, in backend account gets blocked). Another login attempt results in the message
Error
Login denied! Your account has either been blocked or you have not activated it yet.

Or I get the cookie error..

You disabled free registrations. This is going to cause those without an active subscription attempting to login to become blocked. Enable them again within CBSubs > Settings > Global so the accounts stop getting blocked then see if issue persists. For cookie errors please search as we've investigated multiple possible causes and provided multiple solutions.