TokenMismatchException

Today I encountered the following error, and I am not able to resolve it. The error occures when trying to submit a simple login form. Does someone have any idea, where can I begin to look for a solution?

TokenMismatchException in VerifyCsrfToken.php line 46:
in VerifyCsrfToken.php line 46
at VerifyCsrfToken->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in ShareErrorsFromSession.php line 55
at ShareErrorsFromSession->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in StartSession.php line 52
at StartSession->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in AddQueuedCookiesToResponse.php line 36
at AddQueuedCookiesToResponse->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in EncryptCookies.php line 40
at EncryptCookies->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in CheckForMaintenanceMode.php line 42
at CheckForMaintenanceMode->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request)) in Language.php line 40
at Language->handle(object(Request), object(Closure)) in Stack.php line 96
at Stack->Illuminate\Routing\{closure}(object(Request))
at call_user_func(object(Closure), object(Request)) in Stack.php line 82
at Stack->then(object(Closure)) in Kernel.php line 107
at Kernel->sendRequestThroughRouter(object(Request)) in Kernel.php line 82
at Kernel->handle(object(Request)) in index.php line 53