Had the same problem today on iPhone. Didn’t find cause of problem.

I increased my storage (was over 94% used) deleted Zotero from iPhone, restarted phone, reinstalled Zotero from App Store and that error didn’t appear again.

Hope this helps

Everying was smooth since months and suddenly facing same issue from today.

Whenever I edit PDF on my iPad, especially with pencil. It stops syncing the file. It also stops the file getting sync of changes from laptop as well. Basically, making annotations with pencil or highlighting it from iPad causes the error.

I removed all annotations made from iPad. Removed downloaded file and downloaded again. Then I only made changes/ annotated from Laptop and all annotations are synced and viewable on iPad. But, if i make any changes to the annotations or add new annotations, the PDF kind of gets corrupted and stop syncing further to and from the storage. And keeps giving error "SYNC FAILED: FAILED API RESPONSE: POST DATA NOT PROVIDED" and when click on "show item" it points to the specific PDF.

Device:
iPad 9 (1st gen)
iOS 16.7.14

Sorry about that. Please try again now.

It's resolved. You guys are amazing. Thank you so much. Never thought this would be resolved so quickly. Big thank you to the team!!