Hi Davidir ,
First of all, welcome to the Buttonizer Community!
Interesting issue 🤔It does indeed sound more like a server issue than a Buttonizer issue.
After searching a bit I've found this StackOverflow topic. Some are talking about a mismatch between the file size or content length, the content length too big or having not enough disk space to perform the action.
It might be that the content of Buttonizer is too big and nginx was set to handle a smaller payload (that the request fails). You can check the payload in the request via the network tab in the developers tools from the browser. It depends on the amount of buttons and groups you're trying to save.
Personally I haven't seen this issue with this particular error yet, but hope the above topic might help debugging it.
If you found something to fix your issue, please let me know!