Manual:$wgResourceLoaderMaxage

Details
Maximum time in seconds to cache resources served by ResourceLoader.

Default value
To simplify troubleshooting, here is a known exception related to this old syntax:

/load.php?lang=it&modules=skins.vector.styles.legacy%2Cresponsive&only=styles&skin=vector  Error from line 1073 of includes/resourceloader/ResourceLoader.php: Unsupported operand types
 * 1) 0 includes/resourceloader/ResourceLoader.php(891): ResourceLoader->tryRespondFromFileCache(ResourceFileCache, ResourceLoaderContext, string)
 * 2) 1 load.php(51): ResourceLoader->respond(ResourceLoaderContext)
 * 3) 2 load.php(38): wfLoadMain
 * 4) 3 {main}

To fix this issue just migrate to the new syntax without the "server" and "client" distinction.