Just update to Haxe 4.1.0 and the build to HTML5 gave this error:
/usr/local/lib/haxe/lib/openfl/8,9,6/src/openfl/display/Stage.hx:1426: characters 46-59 : haxe.CallStack has no field lastException
Went back to Haxe 4.0.5 and was able to do a build.
Happened to me as well the other day, also downgraded to get around the issue.
Seems in older versions of Haxe, Callstack.lastException exists as a static variable only in JS targets. Guess it was removed in 4.1.0.
This has been corrected in OpenFl develop branch.
Hopefully, an 8.9.7 release is planned soon!