I've tried several configurations of the Helix Library Modern SCM to configure my shared library, all to no avail.

Name: SampleLibrary
Path: //libraries/sample (or //libraries/sample/... as documented, however the logs seem to show this being appended automagically)
Default Version: now (also tried head, master)
Load Implicitly: false
Allow version override: false (or true)
Include in changesets: false

Running this sample pipeline:

pipeline {
    agent any
    libraries { lib('SampleLibrary') }
    stages {
        stage('stage') {
            steps {
                echo 'Library loaded successfully!'
            }
        }
    }
}

results in the following error:

(p4):stop:10
(p4):cmd:... p4 changes -m1 -ssubmitted //jenkins-lib-d94a3a59-55d8-4afb-a69d-9d8da59be94e/...@1___
p4 changes -m1 -ssubmitted //jenkins-lib-d94a3a59-55d8-4afb-a69d-9d8da59be94e/...@1,-1

Invalid changelist/client/label/date '@-1'.

(p4):stop:11
ERROR: P4: Unable to initialise CheckoutTask: com.perforce.p4java.exception.RequestException: Invalid changelist/client/label/date '@-1'.