My file was approved first, but later I received an email saying it violated intellectual property rules and my account was banned. If the design had an issue, why was it approved before?

It’s up to the contributor, not the moderators, to determine if an asset violates IP issues.

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​@daniellei4510 is 100% right.

Contributors assume all legal responsibilities for the content they submit to Stock. This is covered in depth in your Contributor User Guide. 

• https://helpx.adobe.com/stock/contributor/help/submission-guidelines.html
• https://helpx.adobe.com/stock/contributor/help/ip-guidelines.html
• https://helpx.adobe.com/stock/contributor/help/known-image-restrictions.html

Violating Stock’s Contributor TERMS— knowingly or unknowingly— is grounds for permanent account deactivation. Excuses & apologies can’t erase violations. See below.

https://www.adobe.com/legal/dmca.html

Appeals for IP violations are rarely successful. But you’re welcome to try. 

Good luck.

Adobe uses human Moderators who may not be aware of every possible instance of IP. I think that Adobe does routine scanning of the database to look for keywords subjects that violate IP guidelines, and as ​@RALPH_L said, there could also have been a complaint from the owner of the IP.

Can my account be unbanned?

It could be, if it was a violation, that someone saw and reported it.

It’s up to the contributor, not the moderators, to determine if an asset violates IP issues. Moderators aren’t necessarily trained to recognize all of the thousands of logos, designs or even words that are copyrighted or hold trademarks. It’s possible the owner of the copyright issued a complaint, and that will result in an account being disabled while an investigation takes place to determine if there are other instances in a portfolio that contain copyrighted material.