Most likely reasons why only your post fails

Post-specific content triggers a server bug
— e.g., malformed HTML, badly formatted code block, excessive length, or a specific string that causes a template/parser to fail. When the server tries to render that post it hits an internal error and returns 500.

Large or corrupt attachment (image/file) in your post
— very large image or corrupted file can break the image-processing routine and cause 500 only for that post.

Malicious / blocked external link or URL inside the post
— some servers run link-checkers or fetch previews; a particular URL might time out or crash the preview generator.

Database inconsistency for that record (post id)
— the row for that post might be malformed or missing required fields so the app code crashes when loading it.

Permission / moderation / deleted state mismatch
— the post might be partially deleted or in a state the front-end doesn’t handle, causing a crash.

Temporary race / cache / CDN problem
— sometimes an edge cache or the app’s cache has corrupt data only for that resource; others work fine.