After migrating to Joomla 4 on a test site in a subdirectory with a copy of the live site (installed with Akeeba) and reinstalling Extension Manager, Modals Pro en Sourcerer, I get this error message:

Modals cannot function. Regular Labs Library plugin is not installed.
Sourcerer cannot function. Regular Labs Library plugin is not installed.

I tried to open Extension manager, and then I get this error message:

An error has occurred.

0 Class "RegularLabs\Library\Parameters" not found

Also, the live site is broken as well, indicating a database corruption I guess. Before I delete everything and go through the process again (which I already did with the same result) should all Regular Labs extensions be disabled or uninstalled before the migration?
If you wish to see the broken site you may go to: adviescollege-degas.nl/Main/

Funny thing is, I first did the migration on the English language version of my site, without problems.
See: www.advisory-council-degas.com/Main/
Love the new modals, by the way. Updated all of them on that site.

Thank you for your fast reply! That did clear the RL error messages and the RL library was reinstalled.
However, both the live site and the test site are still broken. The test site now gives a single error:
0 - Class "JString" not found

The live site gives a lot of messages:

Fout
mysqli_sql_exception: Unknown column 'a.xreference' in 'field list' in /home/vhosts/adviescollege-degas.nl/httpdocs/Main/libraries/joomla/database/driver/mysqli.php:610 Stack trace: #0 /home/vhosts/adviescollege-degas.nl/httpdocs/Main/libraries/joomla/database/driver/mysqli.php(610): mysqli_query(Object(mysqli), 'SELECT a.id, a....') #1 /home/vhosts/adviescollege-degas.nl/httpdocs/Main/libraries/joomla/database/driver.php(1662): JDatabaseDriverMysqli->execute() #2 /home/vhosts/adviescollege-degas.nl/httpdocs/Main/components/com_content/models/article.php(151): JDatabaseDriver->loadObject() and more like this.

I guess I will have to delete everything again and start over, and uninstall the RL extensions first?

Thanks again and good to know! The remaining problem was indeed a database problem, which was solved after rebuilding the database from a backup. The new Modals are a joy to work with, much cleaner and far more convenient than the Joomla 3 version. For me this alone was worth the migration to Joomla 4.