This looks like the compressed content (GZIP maybe). If you have GZIP enabled for your website (on the server, or through some cache plugin), it can explain this. And the reason for you to see it, might be caused by the output being cut so that it can’t be uncompressed. This can be due to client-server connection failing, some unrelated server issue, or issue with the cache plugin. It is hard to know exactly, but you should check the cache plugin first.