New patch applied now that hopefully fixes the issue. We think the problems is with Spectrum transport and the way ejabberd handles some error messages from Spectrum.
More information: https://support.process-one.net/browse/EJAB-930
We have now applied the patch to the running stable version, it will be fixed in Ejabberd 2.1.4 so next upgrade is safe. Let us just hope this now works! (Thanks a million for the help Asbjørn!)