Jaroslav, unfortunately there is really no way to fix this. When exporting a graphic, the XL Toolbox requests Excel to copy it to the Windows clipboard, then obtains the metafile data from the clipboard to draw a high-resolution image. In the case of the newer chart types, Excel does not provide access to a “Copy” command. Absolutely no way to get around this.
There are other ways to produce high-resolution files from Excel charts though. They require a bit more work on the user’s part. See my blog post:
This is about SVG files, but once you have the graphic in Inkscape (or any other vector graphics software, e.g. Adobe Illustrator), you can export it to a bitmap file (PNG) as well.
Hope this helps at least a little bit!