This problem occurs when updating the installed Eclipse IDE plugin to a new version that includes changes in its support JARs.

Jadescript projects created with the old plugin version could use an outdated JAR which is incompatible with the Java files generated by the compiler included in the new plugin.
The user is currently required to fix manually the projects by copying and overwriting the support JAR files into the libs folder of each defective Jadescript project in the workspace.

An ideal solution to this would include automatic detection and fix of troublesome/outdated Jadescript projects in the workspace.