WiFi Connected But alert "Connected without Internet"

[Gian774]

Dear all,

all Samsung mobile have some issue with our wireless network.
The devices are connected (IP, GW, DNS as expected) but there are an alert "!" with the message "Connected without Internet". Chrome works and Internet is available!

What tests do Samsung devices perform to verify Internet connection? Iphone, Xiaomi,... work without alert.
(Maybe the firewall or URL filter block the test)

Thanks in advanced

Gian

[Gian774]

Yes, the users are able to browse. 
The only problem encountered is with the gmail app (our imap server not working, gmail works) and the user who believes he cannot navigate.

The strange thing is that on thousands of different devices the problem seems to only affect Samsung devices.
(The problem does not occur in a home environment but in several buildings. )

[-Robot-]

That's wierd