I am using Kamailio 5.2. 

Apparently the remapping of 503 to 500 codes in the tm module does also change the to-tag. This behaviour breaks dialogs with yate and therefore calls hang and the 503 remains unacknowledged. After disabling the 503 to 500 remapping with modparam("tm", "remap_503_500", 0) all works fine again.

Changing the to-tag in a dialog seems to contradict RFC3261, or do I see this wrongly?

 Dialogs
A dialog is identified at each UA with a dialog ID, which consists of a Call-ID value, a local tag and a remote tag…"

Thanks for looking into this.

Gerry