I have never seen this error, but that can be a question of configuration options. What I read from the the code, the variable tcl_url2file() is used to cache the mapping from a URL of the requestor to a file only when PerformanceModeP is set (ignoring virtual host mappings, etc.).
My suggestion is to check on the installations where the issue shows up, whether this is true. If so, turn PerformanceModeP (parameter of ACS Kernel) off. If this helps, check, whether there is any noticeable difference in performance (i doubt it).
