So, it turns out we had a live DC in our mirrored DR site which may have been causing some issues for authentication. Also, we had to switch the Zerto Virtual Manager service to run under the same account specified for SCVMM rather than leaving it running under the local system account.
We have installed Zerto on a Windows VM (Hyper-V) in our domain and can browse to the Zerto Virtual Manager Web Client, but for some reason get the following error, regardless of the format (e.g. domain\username, username@domain etc): User validation error – Please try again … try to use fully qualified domain name for login […]