Despite that that the error description indicates the problem is in the "header name," I suspect that the issue is actually with the header_value.
On StackOverflow, The date/time format used in HTTP headers says
In section 14.29 Last-Modified you can see that date format should be:
"Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The last modified date in your error doesn't look like that. It doesn't have any spaces. Instead they have been replaced by
%20 which indicates that the date has been URL encoded.
I don't know of any reason that the standard HTTP headers should be encoded like that. See on StackOverflow: Is it appropriate or necessary to use percent-encoding with HTTP Headers? I'm not sure how you would have configured your server to percent-encode the last-modified header. However you did it, the solution to your problem should be to remove that encoding.
Chrome is probably encountering the first
% and is saying "I didn't expect a percent there! Invalid character."