Cannot open Adobe Express project

Check all these locations for any files that count towards your storage limit:

https://assets.adobe.com
- check also the deleted files section - files are permanently deleted after 30 days;
- check also Launch More>Synced Files - these are folders/files that you can also see on your desktop
https://lightroom.adobe.com
https://documentcloud.adobe.com
https://express.adobe.com/sp/
https://new.express.adobe.com/
https://assets.adobe.com/mobilecreations
- in the Mobile Creations area also check Deleted items.

Here's the help article about cloud storage and what is included as you may use other parts of it e.g. Libraries or Lightroom, that you haven't mentioned here:
https://helpx.adobe.com/creative-cloud/kb/file-storage-quota.html