Hi Bodo-Lutz, please try with this attached version it should boolean union ok now.

I used Edit > Separate and Edit > Join to get rid of the naked edges in the model to make it a solid (although there is still the 0 thickness "pinch point" which may be a problem for other things down the line) and also stretched out the text so that it pushed a little distance down through the model instead of skimming right along it.

- Michael