I sometimes get this USERID_NOT_FOUND ('User not known')

Question

The error specifically looks like this.

[140303 13:39:57.16] [vline.cometdTransport] USERID_NOT_FOUND ('User not known') vline.js:132

I get this error quite some times and have no clue what might have caused this. Have you had this problem? Do you know what might cause this ?

Thanks in advance.