Hello,
I have a problem installing Limesurvey on my Ubuntu Server.

I followed the official (german) limesurvey installation manual (I'm using MariaDB instead of MySQL, but according to github and some forum posts that should work).
But I get an error in the installation process at step 4 (configuration). I am able to create the database and populate it with the tables (checked it) but after I click the "populate" button, it shows "Please fix the following errors: There is no active transaction".
MariaDB itself should work, Nextcloud is running without problems on the same server.

System:
OS: Ubuntu 18.04 LTS 64bit
Webserver: Apache/2.4.46 (Ubuntu)
MariaDB: Ver 15.1 Distrib 10.1.47-MariaDB
PHP: PHP 8.0.3
         pdo_mysql section in phpinfo says "Client API version: mysqlnd 8.0.3"
LimeSurvey: limesurvey3.25.20+210330 (from community.limesurvey.org/downloads/ )

The Pre-Installation check finishes with all minimum requirements checked.
The optional modules PHP LDAP library and PHP imap library are not installed.

I searched the forum and the web and found some people with the same error, but they are all using XAMPP. Their solution was to downgrade to an older XAMPP version, but I can't do this obviously.
I already updated the packages, restarted the server, restarted the installation multiple times, deleted the database and the user - it always shows the same error.

I hope someone here can help me.

I recommend to open a bugticket. To me this sounds like issues around PDO and PHP8.
I wouldn't expect that LimeSurvey is already tested enough to run under PHP 8.
I'm not using PHP 8 before 2022. So I haven't done any tests on my own.

You were right, it was a problem with PHP8.
Downgraded to 7.4 and it works. Thanks a lot, seems obvious in retrospect , but i didn't think about that.