Engine is raising internal server error in case of invalid headers. This might be a problem for older API clients and it's not nice to flood the logs in any case See issue for older clients reported in https://github.com/oVirt/ovirt-web-ui/issues/400
This bug is targeted 4.2.1 but is modified in 4.2.0. Can you please check / verify this bug status and set target milestone and bug status accordingly?
Moving to 4.2.0 as it's part of latest build
Verified with: ovirt-engine-4.2.0.2-0.1.el7.noarch
This bugzilla is included in oVirt 4.2.0 release, published on Dec 20th 2017. Since the problem described in this bug report should be resolved in oVirt 4.2.0 release, published on Dec 20th 2017, it has been closed with a resolution of CURRENT RELEASE. If the solution does not work for you, please open a new bug report.