cannot approve a TA request as admin

[wouteraddink]

Description

If I am logged in as admin and I try to approve a TA request (e.g. if a TAF admin is unable to do so), I get an error:
Something went wrong: Error: Request failed with status code 500

[cannedit]

I will look into this, but I don't think this is a showstopper: in case a TA coordinator (the only role who can approve a TA request, so not a TAF admin) is not available, then an admin can assign the TA coordinator role to some other institution related user.