String is too long ...?

[Rainer Michael Rilke]

Hallo, I am working with a co author on a lyx file. This file is located 
in a dropbox folder. Everything worked out fine for the last month. But 
recently, although I only changed thing in the text (nothing 
substantial) my co author is unable to open the lyx file. He receives an 
error message: "exception: string too long..." What could this possibly 
mean?

Thank you for your help,
Rainer