In several languages the typing of an @ symbol is by using AltGr key. The FileMaker Webviewer on Windows in FileMaker 12, 13, 14 and 15 does not allow you typing anything with AltGr key. At least in my testings.
You can read about the bug on the FileMaker community (here and here). We know FileMaker has probably more urgent bugs to fix, but we hope this gets fixed eventually.
With MBS FileMaker Plugin we can offer a workaround. Our new WebView.PressKey function can be used to send a key event to the webviewer. You can specify which webviewer and pass any key you like to be pressed. Now that is great and you can make a button to press @ key of your user.
If you combine this with our hotkey feature, you can create a hotkey on Windows for alt-q (for German keyboard) to trigger a script or an evaluate. There you can press @ for the user and it appears to the user like the bug is fixed.
This will be available soon with 6.2pr9. If you like to try, send me an email for a test plugin today.