The error messgae "We were unable to complete your request due to a technical problem.", is fairly generic and you will find it multiple times in this thread and the wiki. The "Error code: 519" appears new but there has never been a clear understanding of their meaning
As you have found out, agents for the operating carrier or UA are of no help, these issue are not handle via normal channels and the agents have not access to do anything manual. It will just frustrate you to try so.
The fact there is no I space is not a a positive sign and since there is no waitlisting, probably best to wait for I space before trying again. It is odd UA is suggesting there is space.
These systems are maintain by *A employees, not carrier employees and when they break , which is not uncommon, it can take awhile to get fixed.