It seems that JDK-8278356 which came with OpenJDK 11.0.15 introduces a regression that breaks the JMC build on Windows. The regression is fixed in OpenJDK with JDK-8285445 which will only be part of 11.0.16. For now we can workaround the problem by pinning to OpenJDK 11.0.14 in the GHA workflow which should be fine from a security perspective.