If you're referring to a file sync error, you're receiving an HTTP 400 error from your WebDAV provider. If you can provide a Debug ID (different from a Report ID) for a sync attempt that produces this, we can tell you what specific request is failing, but this is almost certainly a problem with the WebDAV provider. (400 means Bad Request, which is a client error, but Zotero works with properly functioning WebDAV servers.)
<s:exception>TooBigEntity</s:exception><s:message>Posted content length of 525250726 exceeds the limit of 524288000</s:message>
You're uploading a file that's too big (~525 MB) for your WebDAV server (which is returning the wrong HTTP error code for that).
You can paste 7RL594RE into the Zotero search bar in All Fields & Tags mode to find the file in question. If you delete it, remember to empty the trash as well.
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If I want to provide a Debug ID, what can I do?
You can paste 7RL594RE into the Zotero search bar in All Fields & Tags mode to find the file in question. If you delete it, remember to empty the trash as well.