Signature validation takes place within PDF viewing applications and depends on the approach of the corresponding application. Here, no reader proceeds like the second and corresponding standards, such as the necessary check against the European Trust List (EUTL), are rarely adhered to. Some readers with an internal truststore can store the certificate from the EUTL to enable a check against EUTL. If, for example, your Adobe certificates are not up to date, an error message may appear when checking your signature. Instructions on how to update your Adobe certificates can be found here. Acrobat Reader, for example, checks against the EUTL for qualified signatures and also displays this directly with the signatures that have been made. Basically, the validity of the QES is unaffected by the incomplete check in various readers. If a signature is to be verifiably checked, we recommend using the official EU validation site for this purpose, where you can upload and check your document (https://ec.europa.eu/cefdigital/DSS/webapp-demo/validation). This software checks according to the specifications for validation and creates a test report at the end, which logs the process.