Just to say hi and will think about your issue.

As you are using letsencrypt is this your own server?

Please read the post, which details info you could provide as a basis though you provide quite a bit :)

https://forums.b2evolution.net/?disp=edit&cat=244

I will install a fresh 7.2.5 blog on my site to see how it all goes

Exact b2evolution version :7.2.5-stable
Exact PHP version :7.4.33
MySQL/MariaDB version :11.2.2-MariaDB-1:11.2.2+maria~deb12

yes is my own debian 12 install

on ten same server also an update version is running.
www.disconnected-by-peer.at

an parallel i am trying to set up a new installation that owuld replace the old one sometimes in the furue.
blog.disconnected-by-peer.at

the install finished when i set no demo data ...
but then i am unable to login with the admin account with the nice errror from abouth.

Sorry I edited my reply as I saw most of the detail.

I note you have a b2evo site

Sadly I'm very busy for some days, so ??

no need for hurry. but its cracy by the way i have updated my production db to utf8mb4 for thsi change i had to change
evo_i18n_translated_string.itst_standard
evo_i18n_translated_string.itst_custom
varchar to text but its working great as you can see.

so now i think i found out why the login problem the 2 b2evo installs share the same cookie name b2evo so the salt from the on install is different from the other so i had to differentiate the 2 installs by changing this in _advanced.php.

/**
 * Short name of this system (will be used for cookies and notification emails).
 *
 * Change this only if you install mutliple b2evolution instances on the same server or same domain.
 *
 * WARNING: don't play with this or you'll have tons of cookies sent away and your users will have issues!
 *
 * @global string Default: 'b2evo'
 */
$instance_name = 'blog2evo'; // MUST BE A SINGLE WORD! NO SPACES!!

OK That's useful info. I'll get around to trying that sometime.