Verified this bug with the composer build in Advisory RHBA-2021:75056, can NOT reproduce this bug. Composer can handle URL without trailing slash and work as expected.
My steps:
1. Download and install build from https://brewweb.engineering.redhat.com/brew/buildinfo?buildID=1602480
[root@rhel84-iso1 demo]# rpm -qa|grep compose
osbuild-composer-core-28.6-1.el8_4.x86_64
osbuild-composer-worker-28.6-1.el8_4.x86_64
cockpit-composer-29.1-1.el8.noarch
osbuild-composer-28.6-1.el8_4.x86_64
composer-cli-28.14.58-1.el8.x86_64
2. Login cockpit UI, create an blueprint -> create an image -> select RHEL for Edge Container
3. In Repository URL, provide an URL without trailing slash, like http://x.x.x.x/repo
4. UI shows an image is added to queue.
5. Checked the image queue, it shows the image is in process of building, and I checked with command line, it also shows the image is in process of building.
[root@rhel84-iso1 demo]# composer-cli compose status
113361cd-b3ff-49a1-be97-fdb8b4a57084 RUNNING Mon May 17 23:03:33 2021 upgrade 0.0.2 rhel-edge-container
[root@rhel84-iso1 demo]# composer-cli compose status
113361cd-b3ff-49a1-be97-fdb8b4a57084 FINISHED Mon May 17 23:12:29 2021 upgrade 0.0.2 rhel-edge-container 2147483648
6. Try same steps with command line, found no errors, composer can successfully compose the image.
[root@rhel84-iso1 demo]# composer-cli compose start-ostree upgrade rhel-edge-container --url http://192.168.100.1/repo --ref rhel/8/x86_64/edge
Compose ee920a1d-a8fb-4c80-9c6f-7139b28bd762 added to the queue
[root@rhel84-iso1 demo]# composer-cli compose status
ee920a1d-a8fb-4c80-9c6f-7139b28bd762 FINISHED Mon May 17 22:53:43 2021 upgrade 0.0.2 rhel-edge-container
[root@rhel84-iso1 demo]# composer-cli compose image ee920a1d-a8fb-4c80-9c6f-7139b28bd762
ee920a1d-a8fb-4c80-9c6f-7139b28bd762-rhel84-container.tar: 782.84 MB
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (osbuild-composer bug fix update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2021:2585