[NTG-context] reusing figures ok - problem with hiding blocks in MKIV

Dear all, after (finally) installing the minimals, the zip with the figures TeXs as expected with MKII and MKIV. Now, in MKIV I notice another issue. I use the \hideblocks mechanism to hide and reuse answers and get an error message (Undefined control sequence) in MKIV while everything is fine in MKII. I am attaching a sample file. Thanks for all the help! Matthias