So first of all thank you for your tips and effort.

Unfortunately no tip helped. The reason became clear after I had a closer look at "schemabinding".

If you provide functions or views with the option "schemabinding" the tables used there can't be changed anymore.

So I removed this option everywhere and the changes could now be made again using the Table-Explorer, the Management-Studio and T-SQL.

So the problem is solved.
Thanks