Hellooooooo,
youre so right.... i have now deleted these faulty hosts and after a reboot the converter works again.
Next time i look better at the known problems and try to fix it...
Take a Backup.
Go in SQL Studio on the vCenter Server... select the vCenter DB and open a new query window...
select ID from VPX_ENTITY where name ='HOSTNAME';
Now i have a list with my faulty Host-ID's.
Because there are no VM's, Storages or something else i can delete it safetely:
delete from vpx_entity where parent_id = HOST_ID;
delete from vpx_entity where id = HOST_ID;
Restart the vCenter Service.
No Warranty, but it looks very good.
Thank you again for your advice!