Markus and all,
I've been using r.proj and v.proj from pre4 without encountering the problem you report. You might check to see if env[3].value points to the same location each time it is used. I don't know how it might happen, but it is possible that either env[3].value has changed to point to unallocated memory or that the memory pointed to by env[3].value was freed then allocated to some other string.
Roger Miller