To be somewhat productive here: Fatal parser error (at least in my experience) hints at Pulp (the component that does the actualy repo syncing and management in the background) being unable to parse upstreams metadata. This is probably also the reason why you can browse the repos just fine via browser and why newer versions are not affected.
I have found this thread on the forums that mentions similar problem to yours and on a similarly old version. There is a patch linked the you can try to apply to your pulp installation (remember to have some form of backup at hand if this go wrong).
As a general hint at troubleshooting repository problems: Pulp by default logs to both /var/log/messages and Journal. The easiest way to see logs from Pulp would be journalctl -u 'pulp*'. Depending on how many hosts your Foreman manages, this can get quite noisy, but if you have the problem I expect (which is the one I linked above) you should find the “Fatal parser error” in there, followed by a stack trace with something about parsing metadata near the end.