Traveler 9.0.1.15 Build 201611291200_40, Domino 9.0.1FP7, Traveler server added to 'Trusted Servers' field in mail server document. But this error sometime appear again with some changes:

"Error validating execution rights for agent 'wNDROpen' in database '[maildatabase]'. Agent signer '[mail_servername]', effective user '[traveler_servername]'. Examine 'Programmability Restrictions' field in the Server Record."

Did you check the acls to see what the rights of that mail server is on that mail file?

Both [mail_servername] and [traveler_servername] have Manager rights on this database. The same rights are set for other users mail databases. The one difference - 'problem' database have StdR8Mail design, while other DB's have StdR85Mail or StdR9Mail design. I'll upgrade this user  to latest version in next days, then convert this mail db to use StdR9Mail design. May be, it help..

You upgrade the mail file check to make sure what version of the client the user uses.  Use convert with he -u option (for upgrade design of folders)