So, almost all the news on this just repeats the official info released, which does not state the nature of the complaint against him.

However, this one source I found did manage to find someone who would add a little more detail:

"The complaint dealt with a misconduct issue, a source with knowledge of the matter told Task & Purpose. No further information was immediately available."

According to this article:

https://taskandpurpose.com/news/navy-admiral-fired-investigation/