I updated my Arduino environment to latest on github.
Sketches (including WifiManager) wouldn't compile anymore because a recent commit in ESP8266WebServer introduced an enum HTTP_HEAD which was in conflict with a variable in WifiManager (holding the portal header).
I initially renamed this in WifiManager to get it to compile.
The development branch of WifiManager puts these strings in a different file with different names. With this installed instead of the released version then all compiles again OK.