Ran into a strange bug with mobile (different version on 7.6.1.x )I doubt it would be this but can you ensure that he username is all lowercase in the user field. We had a couple users when I was testing it that had mixed case usernames and this caused an issue when trying to log into the mobile apps. Was on SQL with LDAP. For some reason I think this bug was fixed but I don't have something to test this out right now.
Something similar with the DB version
https://community.ibm.com/community/user/asset-facilities/discussion/maximo-mobile-for-eam-app-login-issue