What’s your problem, then?
Cubase 11 can only be opened with the latest eLicenserControlCenter and the dongle plugged in. You knew that beforehand like you said above and it remains true.
Do you want to open a C11 project from within C15 and you are getting an error message? This can be caused by all sorts of things like plugins that still need the eLicenser. Have you checked for vouchers in order to upgrade to the new licensing system in your MySteinberg account?
As others said before - what kind of error message? Without further details we can’t help you other than reconfirm the obvious: Products running on the old licensing system will always require a dongle.
Sorry, it’s difficult to understand what exactly it is you are doing there: Do you open a C11 project from within C15 and this works until you want to close it or are you opening a C11 song by double-clicking on its *cpr? In the latter case, you would still be using C11 which requires a dongle.
Please, post the error message and be more clear about your procedure - what do you open and how do you do it? Otherwise, we can only guess.
I’ve just unplugged my dongle (ahem!) and opened a project from 10 years ago, made with Cubase 8.5, in Cubase 14.. The only errors I have are all for missing plugins.
The project uses HALion Sonic (the paid for version, not the free) and Cubase replaced it with HALion Sonic 7.
Method:
Opened Windows File Explorer
Navigated to my old projects folder
Picked one at random - it is from 2016
Double-clicked the project cpr file
Twiddled thumbs for a minute
Error dialog reporting missing files appeared (the window, not the messagebox)
Clicked OK
Project opened as usual (barring the missing plugins!)
Window title says the name of the project with Cubase 8.5 appended @Slim109 ,what’s your method?
It would be a nice gesture if you’d tell us how the issue was solved and also define the nature of the issue (what did you do to open a project using which version and the error message).
Other users spent their free time trying to help you so that would be a nice thing to do… Moreover, it might help future users who encounter a similar problem.
Cheers!