I think the issue might be that you are using a phrasal verb...sort out. It is much more natural to use phrasal verbs from a native speakers point of view but maybe the person who told you to say "resolved" doesn't think phrasal verbs are appropriate in this instance.
Sometimes phrsal verbs sound less formal.
Other statements similar to yours:
Thank you, I'm glad I could help fix this issue.
Thank you, I'm glad I could be of assistance.
Thank you, I'm glad I could help.
Thank you, I'm glad we could resolve this issue for you.
Thank you, I'm glad we sorted that out.
I think there aren't that many other ways to put it.
Cheers,
Diana