Hello Jan-Willem,
Did you try updating the Marketplace module MendixSSO to the latest version (v4.1.0), which should be compatible with Mendix 10?
Also make sure that you don’t have both old and new versions of the same Java libraries in the ‘userlib’ folder of the project. If you have for example both ‘oauth2-oidc-sdk-9.39.jar’ and ‘oauth2-oidc-sdk-10.7.1.jar’ in the folder, then ‘oauth2-oidc-sdk-9.39.jar’ (plus all related files such as ‘oauth2-oidc-sdk-9.39.jar.MendixSSO.RequiredLib’) should be deleted.