Hi, I think there is a problem with the devices page for the 6620.
It states the 6620 is a MIDP2.0/CLDC1.0 phone.
But I just tried downloading a MIDP2.0/CLDC1.0 jad (with no optional or extra attributes) and it gave a invalid file error. I then changed the CLDC jad atributte to 1.0 and it accepted the jad for install.
After that, I think the 6620 is a MIDP1.0 phone, or at least it only accepts MIDP1.0 downloads via OTA.
Almost all Nokia MIDP2.0 phones are CDLC1.1. But all S60 2nd, symbian version 7.0s are CDLC 1.0. They are: 6600, 6620, 6670, 7610. May be they all have the same problem.
After some research in the discussion boards I found that other developer had the same problem.