See Jetty update, Karaf 4.1.3 upgrade and full LSP support
So I guess, you will have to upgrade to OH2.2.0 #1084 or later. (Snapshot/unstable)
There is no need to fear the “unstable” version as this is the version which gets updates, it’s only “unstable” in the meaning of “new code will be added and sometimes code will be changed”. But only to get rid of errors which preexists in older “stable” versions or at least in completely new features, which are missing in “stable” versions.