OWA not working - OWA-Error: SDServerErr;System.Xml.XmlException

Hi everyone,
for a couple of days users receive the following error when they try to connect to OWA.
The error occured suddenly, nothing was changed at the configuration.
its Exchange 2013 with CU10 and Windows 2012 R2 Servers.
2 CAS and 2 Mailbox Servers within a DAG.

X-ClientId: KQYO - LXOX - ESTK - YVPBVMLG
X-OWA-Error: SDServerErr;System.Xml.XmlException
X-OWA-Version: 15.0.1130.7
X-FEServer: *Server*
X-BEServer: *Server*
Date: 18.01.2016 10:06:35
*url*owa/auth/errorfe.aspx?owaError=SDServerErr;System.Xml.XmlException&owaVer=15.0.1130.7&be=*server*&msg=ErrorUnexpectedFailure
I tried it already with deleting canary data values in the AD. Did not helped.
I have no more ideas for solving this issue. Maybe somebody here could help me.
Thanks.