Thank you for your reply.
I tried to minimize my problem and I found the main reason, but I still do not know what I can do.
My code looks like this
KH := Rationalize[0.12];
L := 1;
S := Sqrt[3]/4*L^2;
\[Mu] := 4*S/(WL*L); \[Phi]0 := Pi;
systemtosolve = { D[\[CapitalPhi][t, \[Xi]l, \[Xi]r], t] == 1 +
Cos[\[Mu] *\[CapitalPhi][t, \[Xi]l, \[Xi]r] +
KH*Re[Z[\[Xi]r] - Z[\[Xi]l]] - \[Phi]0] };
ic = { \[CapitalPhi][0, \[Xi]l, \[Xi]r] == Pi };
vars = {\[CapitalPhi]};
syssolution = NDSolve[{systemtosolve, ic}, vars, {t, 0, 1}, {\[Xi]r, -0.1, 0.1}, {\[Xi]l, -0.1, 0.1}];
NDSolve::deqn: Equation or list of equations expected instead of ConditionalExpression[(\[CapitalPhi]^(1,0,0))[t,\[Xi]l,\[Xi]r]==1-Cos[3/25 Plus[<<2>>]+1250000000/791 Power[<<2>>] \[CapitalPhi][<<3>>]],(((0.
-2. I) \[Xi]l\[Element]Reals&&Re[(0. +2. I) \[Xi]l]<0.208)||Re[(0. -1. I) \[Xi]l]>-0.104)&&(((0. -2. I) \[Xi]r\[Element]Reals&&Re[(0. +2. I) \[Xi]r]<0.208)||Re[(0. -1. I) \[Xi]r]>-0.104)] in the first argument {{ConditionalExpression[(\[CapitalPhi]^(1,0,0))[t,\[Xi]l,\[Xi]r]==1-Cos[Plus[<<2>>]],((Times[<<2>>]\[Element]Reals&&Re[<<1>>]<0.208)||Re[Times[<<2>>]]>-0.104)&&((Times[<<2>>]\[Element]Reals&&Re[<<1>>]<0.208)||Re[Times[<<2>>]]>-0.104)]},{\[CapitalPhi][0,\[Xi]l,\[Xi]r]==\[Pi]}}.
>>
The problem dissappeared if I remove function Z[]
This function is defined like this
Z[x_] := 2*I*Integrate[Exp[-y^2.0 + 2.0*I*y*x], {y, 0, Infinity}];
When I just calculate Z function or draw it's graph everything is fine, but NDSolve do nto like this function somehow.
Can you help me to understand, what is wrong with Z function?
Edited: misprint corrected.
