HTTP2_SESSION_RECV_INVALID_HEADER --> error = "Invalid character in header name." --> header_name = "last-modified:" --> header_value = "Thursday,%2009-Aug-2018%2001:21:53%20GMT"
I'm running 1.8.3. I'm not sure which character is not valid and more importantly, where to find this in the core files (or perhaps an customization was done at some point that did this)?