Oracle released a new version Java 8u144, resolving an issue with the verification of JAR files. (See also previous advisory of openthinclient.)
With this update the openthinclient-Manager can be run without error again.
openthinclient advises its users to update to this version.
More information
8u144 Update Release Notes:
http://www.oracle.com/technetwork/java/javase/8u144-relnotes-3838694.html