SQL-Errors, need help

So, I have the following problem:

Currently, Im running a slightly modded Dolphin version with some custom modules made ages ago locally via XAMPP on a win10 machine. This is working wonderfully, I can log in, create new members etc etc.
On the other hand, getting this to work on our linux-based server is turning out to be a bigger problem than anticipated.  After modifying the header.inc.php with the correct info for the server and uploading it via ftp, trying to access most modules without being logged in results in this:

Not Found

The requested URL /m/groups/home/ was not found on this server.

Also, trying to log in results in a particularly lengthy SQL Error, which seems to indicate that dolphin is trying to access a table or column of some sorts that is not available in the database. Funnily enough, none of those problems exist on the local version, despite using the exact same SQL dump.
Slight edit: Also, we can access the administration panel and see created users, so technically, the connection to the database "should" be established.

Does anyone know a solution to this, or something we can try to work towards? Thanks in advance.

Here is the sql error. I cannot post the backtrace since its over 700k characters and only 180k are allowed in a post, and you probably wouldnt want to read all of that anyway.

The requested URL /m/groups/home/ was not found on this server.

For this one it could be that you don't have permalinks turned on in the Dolphin Admin.  Also, some rewrites are handled in the .htaccess file.  If you are running Nginx, then rewrites are handled differently.  There are instructions for Nginx in the HELP section of the site.

Unknown column 'te.reposts' in 'field list'

This is exactly what it says, the reposts column is not in the database and it is trying to write to it.  You said this was a slightly modded version of Dolphin; exactly what do you mean?  What is the version, please give complete version number X.X.X

One more thing, when uploading to the Linux server, you need to make sure the correct owner/permissions are set for the files.  This is also listed in the HELP section with some shell commands to make it easy.

Permalinks are enabled in the admin panel. Would be weird if they weren't, since it works locally. I dont know if our host uses nginx, we'll have to ask them about that soon.

The dolphin version currently is 7.3.3. It was 7.2.1 before we did all the updates, but even at this stage modules werent working correctly on the server, but on my local machine they were.

With "slightly modified" I meant we have installed a custom made module (the people before me named it "centralised messaging system") and redirect people to this module when they visit the site(you can still access e.g member.php via URL, which then after logging in results in the database error). I dont think it fiddles around with the database at all, but I could be wrong. We're in the process of talking to the original dev.

I indeed know what this database error means, I just have no idea why it exists at all. Everything works locally and we are using the same database dumps on the server as I do locally. Any typical "did you try turning it off and on" style things we could check to see if we made a mistake? Anything we can ask our zero-knowledge-in-dolphin hosts to make them (maybe) figure it out by themselves?

I would created a sub domain for Dolphin Pro, upload a clean version from the current zip, create a database and do a clean install of Dolphin without your custom module.  Test to see if things work as they should.  If so, then dump your database and upload your custom module, import a fresh dump; checking the settings for the dump, and see it it still works.  I have had database exports to get corrupted and importing a corrupted database export means problems.  If after importing your database the site breaks, you know it has something to do with the database.  On the sub domain site, uninstall your custom module and then test.  If things works, then you know there is a problem with the custom module.

Or, you could just uninstall your custom module on the current linux site first and see if the module is the cause of the problems.

Sounds like a page block got left behind during the upgrades over time. Maybe look at the profiles page in page builder in admin...  or what ever page you redirect to after log in.