Hello,
This has something to do with the direct URL created by the plugin for each custom tab (in the “product_tabpage” sub-folder). When the custom tab name as defined by the user is not in English the plugin creates a non-English direct link (by the same name) for this tab, and when then the product editor page fails, if specific non-ASCII characters exist in the tab name.
Manually editing each link name once a new custom tab is created and forcing only English characters in this URL solves the problem.
Cheers,
Nir.