Editing a Member: RoleAssignmentFailed

So I have an umbraco site that uses custom external login (SSO) for members using virtual members here: Virtual members in Umbraco - jeroenbreuer.nl

As far as I know, it shouldn’t have affected the regular members. I’m trying to create a member but for some reason I cannot assign a member to a member group. I’m getting this error: RoleAssignmentFailed. Even simply opening the member and pressing save (without changing anything) gives this error:

Does any one have any idea why I can’t edit? I checked the logs and it says this:

"@mt":"Could not remove roles from member: {errorMessage}","@l":"Error","@tr":"51c6158efc60773e73b95d1ad78e3860","@sp":"4852d7aba9df6186","errorMessage":"The member is not in group ''","SourceContext":"Umbraco.Cms.Core.Services.MemberEditingService","ActionId":"772b91b1-56c6-40b9-afc0-06c627bad9fb","ActionName":"Umbraco.Cms.Api.Management.Controllers.Member.UpdateMemberController.Update (Umbraco.Cms.Api.Management)"

Where can I debug this in the umbraco application? Somehow it thinks the user is in group “” but it isn’t assigned to a group.