Can you tell us more about the system you are running Nextcloud? 32-bit php?
If you push files from your Synology, can you try with smaller amount of data if this is happening as well? Is this a local setup, or could you try to use a test server to upload?
There is a test server (Demo instances of 9, 10, 11 and 12 now live) however I’m not sure if there is enough time to reproduce this error until the server is reset. In this case, I could give you perhaps some credential of one of my test instances.