SP2013 Toolbar keeps loading invalid http json response headers for commandui.ashx

Dear smart people out there,

we published SP2013 via UAG 2010 sp4.

The toolbar keeps showing 'Loading ...' In javascript there is a Deserialization error.

My http response for the commandui.ashx

headers

Cache-Control:private
Content-Type:application/json; charset=utf-8
Date:Thu, 22 Jan 2015 23:01:06 GMT
Expires:Fri, 22 Jan 2016 23:01:06 GMT
MicrosoftSharePointTeamServices:15.0.0.4561
request-id:d860e29c-933b-9069-110a-896f36285dd1
Server:MyWebServer
Server:MyWebServer
Set-cookie:NLSessionSpreauthnappsPersistForOffice=b/aZI5ePhNl6DGrp4hxpUruMARMFDkeeM+jCVbbC5t8CFmeDveN7P0I+ft8OLI02l+rIdu1GcqM7jVj83pDppTC9jtGmKDTc0V9kZg76mxxNddH1fHpxnvAmZwudUxnO;path=/;secure;domain=.company.com;expires=Thu 22-Jan-2015 23:01:21 GMT
Set-cookie:NLSessionSpreauthnapps=b/aZI5ePhNl6EkVp4hxpUruMARMFDkeeM+jCVbbC5t8CFmeDveN7P0I+ft8OLI02l+rIdu15rTM7jVj83pDppTC7jtGmKDTc0V9kZg76mxxNddH1fHpxnvAmZwudUxnO;path=/;secure;domain=.company.com
SPRequestGuid:d860e29c-933b-9069-110a-896f36285dd1
Transfer-Encoding:chunked
X-AspNet-Version:
X-Content-Type-Options:nosniff
X-FRAME-OPTIONS:SAMEORIGIN
X-MS-InvokeApp:1; RequireReadOnly
X-Powered-By:
X-Powered-By:
X-SharePointHealthScore:0

body

bbe8
{"name" : "CommandUI","attrs":                
{"xmlns":"http:\u002f\u002fschemas.microsoft.com\u002fsharepoint\u002f"},children:
[{"name" : "Templates","attrs": {},children:[{"name" : "RibbonTemplates","attrs": 
{"Id":"Ribbon.Templates"},children:[{"name" : "GroupTemplate","attrs": 
{"Id":"Ribbon.Templates.ManageViewsGroup"},children:[{"name" : "Layout","attrs": 

... more content ...

{"Id":"Ribbon.Tabs.InfoPathListDisplayTab","Command":"InfoPathListDisplayTab","Description":"","Title":"View"}}]}]}]}
0 

Did you ever encounter a similar problem? It seems this problem started when upgrading UAG to SP1.

All advice is welcome!

Thanks in advance, AllWorkNoPlay