I'm glad I'm not alone but is there not yet an answer to this problem?!
I am using Monterey 12.2.1, Transporter 1.2.3.
Same problem, Transporter appears to freeze when UPLOADING APP when I am uploading any large (for example my TFFast app is 200MB, my little apps go up fine). Then my whole Mac freezes up. I either hard-reboot (no response from Force Quit) or if you leave the Mac a while, it reboots and gives the following message when reopening Transporter:
Package (s) cannot be found. The following package could not be found: /var/folders/.... 1217478123-01-038.156.01-1.itmsp. They will be deleted from the queue.
iCloud has been updating very slowly recently?! Could there be a relationship between ability to upload using Transporter and iCloud sync status?
Please help us Quinn the Eskimo!