I'm really sorry that you are still having trouble with this! The error means that the server is returning a 400 error. This usually indicates that the server believes there was something incorrect about the submission but with what you've described it does not sound likely to be an issue with Collect. You could ask the Ona support team to specifically look into when Ona returns a 400.
Are there submissions from the same form version that make it onto the server? Are there submissions from the same device(s) that have failures that make it onto the server? Any more information you can provide in terms of patterns you see will help with troubleshooting.
How widespread is this? E.g. 5% of submissions vs. 40% of submissions. Does it affect certain devices/data collectors more? Is the pattern of many repeats consistent?
If you have access to another server you could change the server address and send to that, then consolidate the data as you need. That would also help rule out an issue with Collect. Or you may find it easier to get the data off the devices manually following the instructions at How to upload offline data on the android phone to the ODK server if the server has changed but the forms are the same - #4 by yanokwa.