Good news from the eZ camp: they finally fixed bug #6199. The bug is triggered by enabling the TemplateOptimization setting, which tries to optimise some calls made in compiled templates.

If you assign a content node to a variable called $node in templates, calling $node.object.data_map may fail. The optimisations assume that $node is only used by the content module and not by custom modules or any other templates.

The bugfix will appear in the next point releases who should be released quite soon.