Strange vertical space with svmult and subequations environment

When using the class svmult and the environment subequations from amsmath, an ugly vertical space is inserted between text and formula, when a label command is used. Moving the subequations env. in front of some text, it causes a line break.

documentclass{svmult}
usepackage{amsmath}
begin{document}
  Some text. 
  begin{subequations}
    label{eq:NNMex1Ham}
%Moving the label command to a previous line, introduces a line breaks.
  We consider the resonant oscillator
  begin{align}
    ddot{x}_1 + x_1 + x_1^3 + K (x_1-x_2)^3 &= 0,
    ddot{x}_2 + x_2 + x_2^3 + K (x_2-x_1)^3 &= 0,
  end{align}
  end{subequations}
  which will occur also in the next chapter.
  

  We consider the resonant oscillator
    begin{subequations}
    label{eq:NNMex1Ham2}
  begin{align}
    ddot{x}_1 + x_1 + x_1^3 + K (x_1-x_2)^3 &= 0,
    ddot{x}_2 + x_2 + x_2^3 + K (x_2-x_1)^3 &= 0,
  end{align}
  end{subequations}
  which will occur also in the next chapter.

  %Now the environment without a label command
  We consider the resonant oscillator
    begin{subequations}
  begin{align}
    ddot{x}_1 + x_1 + x_1^3 + K (x_1-x_2)^3 &= 0,
    ddot{x}_2 + x_2 + x_2^3 + K (x_2-x_1)^3 &= 0,
  end{align}
  end{subequations}
  which will occur also in the next chapter. 
end{document}

The solution AtBeginEnvironment{subequations}{ifhmodeunskipfi}found in the answer to a very similar question also solves the problem, but in the current case it cannot be applied: I am one of the authors and have no access to the editor’s style files or header lines.