Thanks, Frank for that hint. At first I thought this explains my mistake. I was misleaded by the documentation which tells:
FindInstance[expr,vars]
finds an instance of vars that makes the statement expr be True.
But within the details-section they explain that expr has to be equations, etc. But within the examples there are some with pure logical expressions, not equations.
Anyway. If I write it as an equation, I get another error and ridiculous results:
In[206]:= FindInstance[{n > 0, Divisible[n, 37] = True}, n, Integers, 5]
During evaluation of In[206]:= Set::write: Tag Divisible in Divisible[n,37] is Protected. >>
Out[206]= {{n -> 31}, {n -> 33}, {n -> 267}, {n -> 335}, {n -> 400}}
If I write it with "==" instead of "=" I get:
In[205]:= FindInstance[{n > 0, Divisible[n, 37] == True}, n, Integers, 5]
During evaluation of In[205]:= FindInstance::exvar: The system contains a nonconstant expression True independent of variables {n}. >>
Out[205]= FindInstance[{n > 0, Divisible[n, 37] == True}, n, Integers,5]
I'm afraid, I still have some basic misunderstandings of the Wolfram Language.