And the culprit was...

Proguard! It's a bit my fault: I forgot to tell that "context", a long
value in Rexx.java, should not be obfuscated.  But, even after
this change + new proguard CFG structure, the problem reappeared.

No proguard, no problemo.  Proguard is less than 1% of footprint improvement.
Why would I to bother for this ridiculous amount? I'm not even trying to
spend one second at fixing my Proguard config.  That was enough a pain
in the neck, and I just don't care.

This is branch b2, soon to be merged back to Emerald.
Rexx error messages described on the Java side (toast)

The challenge was to percolate messages from the C++ core up to Java.
This was just not implemented.