The bug tracker is offline, but just in case you see this Nuxwin , this issue is back with latest LetsEncrypt 3.6.0
I do not remember the solution you did apply, as this was fixed, but this is the exact same issue.
If I disable the Let's Encrypt plugin, this is working, failed again after re-activated.
Well, that's always the same workaround as I said there : PhpSwitcher: add APCU, Imagick and IONCUBE 
It's already set to use PHP7.1, funny so.
One day I'll take a better look at it, thanks.
The content of /var/lib/nginx/fastcgi is empty, and I already restart the imscp_panel service (if not done, can't access any PMA page).
That's really odd, really sounds like a cache issue, but don't know where it lies (but as far as PMA 4.9.x is updated, it's fine... for now)
Speddy I see you did upgrade to the PhpMyAdmin 5.x series.
When I did it on two servers, it's working, but there is "a sneaky bug" with me.
Can you go when logged (admin or user), on any database (left menu), inside the "new table" link (in your screen page 3, would be the "Neu" link, the URL you should get is of this form : "https://YourPanel.tld/pma/tbl_create.php?server=X&db=DBName") ?
When I click on it, infinite loading, nothing came to me (just the URL change, if I refresh, get a long error).
So far, I'm stuck with release 4.9.x 
No support will be provided to outdated system (update your i-MSCP panel).
Firstly : You want some kind of support ? Well, wrong location for this post, should be under "support" section
Secondly : You still want some kind of support ? Well you should follow the reporting rules written there : Reporting rules - Reminder
Third and last for now : "flames" asked you THE SERVER'S LOGS about the error, why you stick with the browser reports which are useless there ?...
So now, comply with the rules or thread will be closed... (you are not new here, you should be aware of those rules)
Having a big breach inside your server could lead to an even more massive outage of your server.
Just saying.
"E: Unable to correct problems, you have held broken packages."
Not an i-MSCP issue, you might have some broken update/install with APT.
Fix it and re-run the setup.
