... as I did a "Reset" and a new scan in the vst-plugins.
I had to do a "Reset" because a vst-plugin was not found in the list.
 
Now: 
Track with "Cakewalk Session Drummer" loads the Native Instruments Kontakt instead
Track with "Addictive Keys" loads Kontakt instead.
Track with "Play" (EastWest) loads the Session Drummer instead
and so on ...... 15 instruments are changed into an other instrument.
These are all "simple instrument tracks"
 
I thought that SONAR identifies the vst-plugins based of a manufacturer-ID-number and the name of the plugin.
But that everything is totally interchanged after a "Reset" should not happen.
 
 

Try a reset and rescan again. 

I'm running 2017.03 with no problems, so I think doing a reset and rescan again is probably a good idea.

I have done a couple of vst reset and scans with 2017.3 and definitely did not have an issue with instruments changing.

That sounds... interesting! Never seen that one. Time to get some R&R! 
 
(meaning Reset and Rescan again, same as the others said)
 
Also, you are very correct that it should not happen, and I hope you get it sorted!

I have done a "Reset" and tried to done a new scan.
But since a hour the scan is running in a loop. That means the dialog shows all second a plugin and after 1 minute it shows the same plugins again and again and again.
There must be a very mysterious mistake.
Now it's late at night (in germany) - and I'll go on tomorrow.

Check there's not a dialog box behind a screen waiting for input.

I don't think this is a thread-jack, because I have a similar issue. For me, it's mostly an annoyance. When I finish working on one project, close it, then open another without closing Sonar, I often have the same problem--the new synths I try to insert are replaced with something else. I was going to post here and ask if anyone else had this issue and then saw this thread.
 
I hope it's not a harbinger of worse things to come? Or perhaps a result of having too many plugins? I use Plugin Manager and have always created my own plug-in menu. Is that a factor, I wonder? Is whatever causing my problem also causing the OPs problem?
 
At any rate, it's only an annoyance for me. When I quit and restart Sonar and everything is fine again. It's just weird.

Now I did a new "Reset" and a new "Rescan".
It took 6 hours and often stops and hangs. I restartet several times the computer and made every time a new scan (scanning failed plugins).
Sometimes the computer doesn't react when I click "Scan". Then I closed SONAR, restartet the computer and then it does.
Now the instruments are in the right place.
 
I never had such a problem in the last 10 years scanning vst-plugins.
I've noticed that the scan asks for licences which I already entered several times.
 

THambrecht
...Sometimes the computer doesn't react when I click "Scan". Then I closed SONAR, restartet the computer and then it does.
Now the instruments are in the right place.

 
Hmm...I don't know if you should be so quick to mark this thread solved because now your problem is starting to sound similar to mine. (I hope not! I hope you are all fixed!)
 
However, keep an eye on what happens when you work on one project, close it, then open another file without closing Sonar. That's when my VSTis start getting confused. Not on those that are already in that second project but any new ones I try to insert. Closing and reopening Sonar fixes it.