As per guidelines I posted this issue in the user list here but no one was interested in helping me verify it.
There seems to be an issue with the application/vnd.ogc.gml/3.1.1 info_format on WMS GetFeatureInfo requests in 2.14.1. When the "/3.1.1" suffix is included the GetFeatureInfo request hangs indefinitely and no errors are logged. An identical request without the suffix returns as expected.
"/3.1.1" is URL encoded and the full argument string is &INFO_FORMAT=application%2Fvnd.ogc.gml%2F3.1.1
I can replicate this issue with a Shapefile-based layer as well as an Oracle-based layer with the following workflow:
Open OpenLayers Layer Preview
Open browser's Dev Tools
Click on map
Copy GetFeatureInfo URL from Dev Tools Network tab
Replace info_format argument with application%2Fvnd.ogc.gml and issue request in new tab (works)
Replace info_format argument with application%2Fvnd.ogc.gml%2F3.1.1 and issue request in new tab (hangs indefinitely)
The User Manual says that this info_format argument is valid in 2.14.x. The manual even suggests using this format: "For maximum control and customisation the client should use GML3 and style the raw data itself."