See Knowledge Base Entry 1LKEC6G4. It says:
"If you are creating the file path by converting from a string, you may have inadvertently included control characters such as tab, carriage return, or newline in the string that is used to create the input path. Even though in you cannot see control characters in your input string, they may be present. In order to look for control characters, examine your input string in a string control or indicator with backslash code display enabled (pop up on the control or indicator and select "'\' Codes Display"). If you see "\n" or "\t" or "\r" then you have unintentionally included a newline, tab, or carriage return in the string to be converted to your path. As of LabVIEW 5.1, control characters are not stripped from strings prior
to conversion to paths."
Also, make sure you have free disk space.
If the answers you have received don't solve the problem, you could attach a small VI that reproduces the problem. Make sure to indicate the version of LabVIEW that you are using.