I’ve uploaded many photos to Adobe Stock, but I keep getting the “Similar Content” rejection. How can I fix this problem? What should I change in my images or workflow to avoid similar-content rejections? I’d really appreciate your advice.

Focus on making your images unique. Making an image unique is no guarantee you won’t get a similar image rejection but you increase your chances you won’t. For simplicity purpose lets say you have an image of a rooster. Obviously there are tons rooster images. But are there tons of a Rooster images ringing a farm bell by tugging a rope with its beak. I think it helps to include some kind of action the subject is doing. In this example the subject is the rooster and the action is ringing a farm bell by tugging a rope with its beak. Basically you are telling a story with your prompt. I keep repeating myself  but the story is a rooster ringing a farm bell by tugging a rope with its beak. 

Another example. Two girls are having a snow ball fight. Subject is two girls and the Action is a snow ball fight. Girls and snow ball fight is probably pretty common But you can try to make it unique like this:  one girl is hiding behind a snowman and using it as a shield while the other girl launches a snowball from behind an overturned red sled. That makes it unique. 

There are no guarantees but this process has cut down my similar image rejection to once in a blue moon.  
 

Here are some subjects you should definitely avoid: dogs, cats, wallpapers and backgrounds, and flowers, to name a few.

Before you submit, use advanced Reverse Image Search to find similar content.

https://stock.adobe.com/search-features

Rejection due to similar content applies not only to the asset's actual content but also to its title and keywords.
If a large number of assets with the same titles or keywords are already registered, your asset may not appear in search results, preventing sales.
You need to assign distinctive titles and keywords that reflect the asset's unique characteristics, or carefully consider the theme itself when creating the asset.

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-distinct-content/distinct-content-submission-best-practices.html