SSLVPN can not access OWA from Desktop

[nknit]

Hello,

since update 6.4.4 FortiOS we have a problem to connect to owa from desktop when using SSLVPN webmode. Login to SSLVPN is possible. The bookmark to OWA will load the login page for OWA. After login OWA wil show short "loding inbox" and then we get the error page "Somthing went wrong". The error is "X-OWA-Error: SessionDataError;SrvErr:null,ClientErr:JsonParser is not defined,ValidResponse:true"

We use:

Forigate 101E 

FortiOS 6.4.4

SSLVPN Webmode

Exchange 2013

When we connect to OWA with SSLVPN over smartphone, all will be ok. We get the inbox an can read e-mails. Only Desktop will get an error. I've tried this wit Chrome, Firefox, Microsoft Edge an Windows 10 and Firefox an Ubuntu 20.04.

We use SSLVPN and OWA for long time, there no erros until update.

Can someone help with a hint what changed with SSLVPN after Update to 6.4.4? We have upgraded from 6.2.2 to 6.4.4. I don't know if it's important to know.

Thanks

Markus