It happens from the time to time on Chrome-based Chrome-based and Firefox browsers (fully updated versions, clean installs, also in incognito modes), on some (but not all) Windows machines.
Example of called URL is https://2.base.maps.api.here.com/maptile/2.1/maptile/2f447c1cac/normal.day/11/1071/663/256/png8?xnlp=CL_JSMv3.0.17.0&app_id=*********&app_code=******&lg=pol&criticality=major%2Ccritical
The status of the connection is "Pending" (but 200 OK) for 30 secs, then ends with no response (still 200 OK). But the console is reporting net:ERR_SPDY_PROTOCOL_ERROR 200 OK.
As a result tiles aren't loaded, map does not work as expected. The license is commercial (not freemium one).
A log from chrome://net-export/ and parsed by netlog-viewer, contains descriptions like this:
https://1.base.maps.api.here.com/maptile/2.1/maptile/2f447c1cac/normal.day/11/1075/662/256/png8?xnlp=CL_JSMv3.0.17.0&app_id=******&app_code=******&lg=pol&criticality=major%2Ccritical
Start Time: 2019-04-01 14:29:29.999
t= 0 [st= 0] +REQUEST_ALIVE [dt=30043]
--> has_upload = false
--> is_pending = true
--> load_flags = 268452672 (BYPASS_TURBO | DO_NOT_SAVE_COOKIES | DO_NOT_SEND_AUTH_DATA | DO_NOT_SEND_COOKIES | MAYBE_USER_GESTURE)
--> load_state = 15 (READING_RESPONSE)
--> method = "GET"
--> net_error = -1 (ERR_IO_PENDING)
--> status = "IO_PENDING"
--> url = "https://1.base.maps.api.here.com/maptile/2.1/maptile/2f447c1cac/normal.day/11/1075/662/256/png8?xnlp=CL_JSMv3.0.17.0&app_id=*****&app_code=*****&lg=pol&criticality=major%2Ccritical"
t=30043 [st=30043] HTTP2_STREAM_ERROR
--> description = "Server reset stream."
--> net_error = "ERR_SPDY_PROTOCOL_ERROR"
--> stream_id = 1337
t=30043 [st=30043] -HTTP_TRANSACTION_READ_BODY
--> net_error = -337 (ERR_SPDY_PROTOCOL_ERROR)
t=30043 [st=30043] FAILED
--> net_error = -337 (ERR_SPDY_PROTOCOL_ERROR)
t=30043 [st=30043] -REQUEST_ALIVE
--> net_error = -337 (ERR_SPDY_PROTOCOL_ERROR)
The issue was solved by HERE Maps.