Might have to do with your MX 5.21.10 version. See the releas notes of MX 5.21.7:
‘Please note that after upgrading to Mendix 5.21.7, it will not be possible to upgrade to Mendix 6.10.7, which is currently the latest version of Mendix 6. To migrate a 5.21.7 project to Mendix 6, you will have to wait for the next Mendix 6 release, which is due in a few weeks.’
Try converting to 5.20 instead of 5.21, after this you should be able to go to Mx 6
That issue comes up when you try to upgrade to a version that was released before the current version you are in. In this case 5.21.10 was released on 7/30/18 so you want to find a version that was released after that. Looking at the app store , I see that 6.10.16 – 6.10.18 fall in this category. I would try upgrading to one of those, and then do the same thing for the Mx7 version. If you were to upgrade to 6.10.16 then you can upgrade to 7.19.0 and above.
Hope this helps!
Edit: You can check out this thread https://forum.mendix.com/link/questions/14842 but looking at it, you might need to create a support ticket.
I have to migrate without the WSDL and than import the (modified) WSDL again.
The only way that i could migrate to Mendix 7 was to remove all the functions request en anwers for the WSDL. Than upgrade to Mendix 6 and Mendix 7. Build all the request en answers again from a modified WSDL (I removed some of the choices).
In order to prevent problems in the future, I also build a layer between the WSDL and the rest of the applications that use the Mendix app to connect to a third party.
Thanks for all the support.