Hello 1jan,
Thank you for using Apple Support Communities! It looks like you haven't received an answer to your question. If you are able to provide additional information, then perhaps someone will be able to assist you. Some questions that might be helpful to answer are:
1. Does this error appear with a specific email account or any email account? If a specific one, who is the email provider?
2. Do you use a VPN or do you have a profile installed on the iPad
3. Does the behavior occur when connected to different Wi-Fi networks?
Cheers!