This issue was caused by the upload of Ant 1.9.8. In this version the systemClasspath field in the Path class was made final [1], and the InternalAntRunner class in Eclipse attempts to modify it [2], leading to the IllegalAccessError that can be seen in the .metadata/.log file.
Most eclipse based packages are potentially affected. I plan to revert the modification in Ant and make the field non final. Emmanuel Bourg [1] https://github.com/apache/ant/commit/984a03d [2] https://github.com/eclipse/eclipse.platform/blob/R4_6_maintenance/ant/org.eclipse.ant.core/src_ant/org/eclipse/ant/internal/core/ant/InternalAntRunner.java
