I found two situations in which it is possible try to burn
a multissesion disc using a invalid imported session:

insert a multissession rewritable disc
create a multissesion project and import the session from the disc
add some new file(s) to your project
now click "Format" button in toolbar and erase your disc
after erase process, your disc is ejected. Re-insert it and click "burn" button
Click "burn" button again to start burning process
I get the dialog from my screenshot.

OR

insert a multissesion disc
create a multissesion project and import the session from the disc
add some new file(s) to your project
eject actual disc and insert another disc
Click "burn" button to start burning process
Again I get the dialog from my screenshot.

The problem here is that the message from the dialog is not clear and does not explain
the situation. For example, I thought that it was a bug.
https://bugs.kde.org/show_bug.cgi?id=387384#c14

In my screenshot you can see "Searching for old ses".
But this message is hard to see and cut off.

Maybe K3b should prevent the use of an invalid imported session to burn a disc
or show a better message in the dialog to clarify the situation.

I use K3b 17.08.3 on Arch Linux.