Tomcat Bad-Response (White page) for few users

I got a Problem with my Java Webpage. I just allow access over Port 443/HTTPS which is unblocked by the Firewall.

For most of the users the access to the page works fine: Accesslog:

"GET /test/ HTTP/1.1" 401 994
"GET /test/ HTTP/1.1" 200 2176
"GET /test/VAADIN/widgetsets/com.vaadin.DefaultWidgetSet/com.vaadin.DefaultWidgetSet.nocache.js?1490798129908 HTTP/1.1" 200 3295
"POST /test/?v-1490798129909 HTTP/1.1" 200 10346
"GET /test/VAADIN/themes/valo/fonts/open-sans/OpenSans-Light-webfont.woff 
HTTP/1.1" 304 -
"GET /test/VAADIN/themes/valo/fonts/open-sans/OpenSans-Regular-webfont.woff HTTP/1.1" 304 - 
"GET /test/VAADIN/themes/base/fonts/fontawesome-webfont.woff HTTP/1.1" 304 - 
"GET /test/VAADIN/themes/valo/fonts/open-sans/OpenSans-Semibold-webfont.woff HTTP/1.1" 304 -
"GET /test/VAADIN/themes/base/fonts/themeicons-webfont.woff HTTP/1.1" 304 -

Now somehow some users just gets a Blank-Page with the Tomcat Favicon:

"GET /test/ HTTP/1.1" 401 994
"GET /test/ HTTP/1.1" 400 -
"GET /test/ HTTP/1.1" 401 994
"GET /test/ HTTP/1.1" 400 -
"GET /test/ HTTP/1.1" 401 994
"GET /test/ HTTP/1.1" 400 -
"GET /test/ HTTP/1.1" 401 994
"GET /test/ HTTP/1.1" 400 -

Tomcat Version 8.0.41

Vaadin Plugin V3.0.0 in Eclipse