since my song memory is full, I started importing them to Cubase. I have always saved all of my songs with the performance linked to it, and since the new update, most of those links are either gone or wrong.

The update should not have disturbed any of the relationships between Performances and Songs, because you should have made a Backup file prior to the update. Restoring that Backup file (.X7A) would restore all of your settings, Songs, Performances, Libraries, etc.

So most of the songs are useless because of course, I don´t remember the performances... pretty depressing.

That should not happen because, of course, you properly saved your work prior to performing the update, right? Of course.

and also, when importing a song with Montage Connect, Cubase always creates a "tempo track" which is active by default and has the wrong tempo, and one song even was jumping around between 120 and 80, which I of course never recorded like that in the Montage because that would be impossible.

How do you have MIDI Clock set? In MONTAGE? In Cubase?
The original Tempo is stored in your MONTAGE Performance. It can be stored in the MONTAGE Song... when you are originally creating the data— there is a Store Tempo box.
When you import it to Cubase that is when you ensure that you make Cubase the Tempo Master. It’s default Tempo is 120 bpm, and I bet your Performance Tempo is 80 bpm. Your mystery is caused because you have not setup the MIDI Clock communication.

Nothing weird is happening... it only seems like it is all happening to you.

yes, I did a backup file, but at least half of the songs are not linked to a performance. I am sure I always saved them with the performance, because there´s not much point if this is not done. but it could be that those songs without a linked performance are user arps... but how can I tell them apart when they´re all listed as songs?

and why does the Montage always jump to the piano sound after importing a song? it´s kind of annoying. maybe can be fixed in the next update.

the midi clock is always set for Cubase to be the Master. I don´t ever deal with that Cubase tempo track, but it seems like the tempo of the imported song is on that tempo track, is that right? I guess the one that jumped from 120 to 80 on the tempo track is corrupted. How could I possily have recorded a tempo change on the Montage? I certainly wouldn´t know how to do that, nor do I think it´s possible.

yes, I did a backup file, but at least half of the songs are not linked to a performance. I am sure I always saved them with the performance, because there´s not much point if this is not done. but it could be that those songs without a linked performance are user arps... but how can I tell them apart when they´re all listed as songs?

Song are in the “Song” Folder, Arpeggios are in the “Arp” Folder. What do you mean how can you tell them apart? Songs start when you push the Play button, Arps start only when the Arp On/Off switches are activated and you trigger a key in the Note Limit defined. (?)

and why does the Montage always jump to the piano sound after importing a song? it´s kind of annoying. maybe can be fixed in the next update.

It does so because you haven’t instructed it to do anything else. That’s how stuff works... there is a default setting until you tell it to do something different. When you turn On your MONTAGE why does it always boot up on the Preset LIVE SET? Because that is the default, and you have not instructed it to do something different. That’s what ‘default’ kind of means.

If you are using “Song Import” to capture your MONTAGE Song, it can capture the current Performance, as well. So the smart user will have the correct Performance called up when they import the Song data, so they tap the “Recall” icon that directs the Keyboard >. Computer... this action captures the Performance. No update needed, again, it’s you, I’m afraid.

There are two components when moving data into Cubase with MONTAGE CONNECT... the sequence data (MIDI events) recorded to the Tracks - these get imported via the “Song Import” function... then there is the Performance capture (called “Recall” in CONNECT) - This moves the Performance settings to the computer.

the midi clock is always set for Cubase to be the Master. I don´t ever deal with that Cubase tempo track, but it seems like the tempo of the imported song is on that tempo track, is that right? I guess the one that jumped from 120 to 80 on the tempo track is corrupted. How could I possily have recorded a tempo change on the Montage? I certainly wouldn´t know how to do that, nor do I think it´s possible.

It not only is possible obviously you either figured how to do it by accident (you’ll need to tell us how you did that)! Or as you said, you “don’t ever deal with that Cubase Tempo Track”. Well, it will always deal with you... in order to run Cubase must deal with Time Signature (you likely don’t deal with that either, good thing you are using 4/4) and Tempo, which as mentioned defaults to 120.

Suggestions:
If you take the time to record a Song, Pattern, Arpeggio, or Performance, take the time to NAME it.
When you create a Song or a Pattern, take the time to Store “Song&Perf” or “Pattern&Perf” so that they are paired to each other. If your Songs just have Numbers please give it a Name... most people name and store the Performance and the Song similarly so that it is obvious which goes with which, make sense?

well, about that tempo track: when I directly record into Cubase, I set the tempo and that´s that. No need to activate the "tempo track" unless I want to edit it and draw in tempo changes. What else is the point of the tempo track?
But when importing a song, then I noticed I have 120 as the Cubase tempo and a different tempo when activating the tempo track. So I realized the tempo of the song recorded on the Montage ends up on the separate so called tempo track. A track I try to avoid and not deal with because having 2 different tempos and having to remember which one is the right one certainly only adds to more confusion, if not Cubasing every day, one tends to forget some of these settings....and then one is stuck again wondering what is wrong with the tempo...and why midi and audio tracks don´t match....

update: actually, I don´t even have a tempo track in Cubase Elements 10...but a tempo editor. anyway, I got 2 different tempos regarding wether or not a little miniature icon next to the tempo field at the botton right is active or not. that´s what I mean. very very easy to overlook and get problems....