Did you enable the pdfviewer extension? The pdf preview is now an extension.
Hard to tell, what is going on here. You can alternatively just skip the installer and go to index.php. If you use sqlite3, you will not even have to setup the database.
How did you do the update? Did you notice that the directory pear no longer exists and vendor is now in th root dir? Is extraPath in settings.xml still set?
The format of the csv file is wrong. Do an export of the users on the 'List of Users' page. Take that file as an example.
I strongly discourage anybody to use such a service. It doesn't tell on its web page where it is located and who is running it. It also makes little sense to convert the documents before importing them into SeedDMS. If you want to do that use LibreOffice. The best solution for SeedDMS is currently https://codeberg.org/SeedDMS/unoserver
This will not work, because the internal id for each record will conflict. Did you use workflows or custom attributes? If not, you could try to just copy the content of one seeddms installation into the other one by webdav. You will loose some meta data, but maybe that's not a problem in your case. The other way would be to use the utilities xmldump and xmlimport in the utils directory.
Rebuild the fulltext index after adding the new attribute. This is a known problem when you use sqlitefts. Anyway, I still recommend to use sqlitefts. I'm working on a fix for it.
The current seeddms version already contains code to index the meta data of attachments. Have you noticed that?