unable to accept a review on a release request with accept_at set

[andreasstieger]

Request: #412009
    *** This request will get automatically accepted after 2016-07-19 21:19:31 UTC ! ***

  maintenance_release: openSUSE:Maintenance:5372/yast2-network.openSUSE_Leap_42.1_Update -> openSUSE:Leap:42.1:Update/yast2-network.5372
  maintenance_release: openSUSE:Maintenance:5372/patchinfo -> openSUSE:Leap:42.1:Update/patchinfo.5372
[...]
State:   review     2016-07-18T21:19:31 AndreasStieger
Comment: <no comment>

Review:  new        Group: opensuse-review-team                                                           
         accepted   User: openqa-maintenance                           2016-07-18T21:19:33 openqa-maintenance    
  ok

History: 2016-07-19T09:21:00 openqa-maintenance Review got accepted
         2016-07-18T21:19:33 AndreasStieger Request created
[...]

Then:

 osc review accept -G opensuse-review-team 412009 -m "approve SLE imported update"
Server returned an error: HTTP Error 500: Internal Server Error

Request: https://api.opensuse.org/request/412009?newstate=accepted&by_group=opensuse-review-team&cmd=changereviewstate
Headers:
status: 500 Internal Server Error
x-request-id: fe9dd16c-b07e-44d7-9895-21f0049be3a6
x-powered-by: Phusion Passenger 5.0.22
connection: close
strict-transport-security: max-age=31536000
content-length: 116
server: Apache/2.4.16 (Linux/SUSE)
x-runtime: 0.547432
date: Tue, 19 Jul 2016 15:50:57 GMT
content-type: application/xml; charset=utf