Hi Mirek,
I do use namespace in my application, but I do not use something called 'Ref'.

The error of the compiler actually says that the ambiguity is between 'Upp::Ref' or 'Eigen::Ref'.

By the way, I think I have fixed the problem. In my application the include of the SCHEMA file and related headers (ref. https://www.ultimatepp.org/srcdoc$Sql$BasicUse_en-us.html) was very messy: there was an include file who collected some other headers, but in some case this was included in a third header. I didn't care very much till today, 'cause everything was working. So I cleaned up the mess and it came up that the problem was the following line:

#include <ScatterCtrl/ScatterCtrl.h>

If I include the above header before the header related to the schema file, I got the error, if I put the schema before and then the ScatterDraw it works fine.

Bye,
gio