I have this problem too and I think you will find others are also. I'm sure Smith Micro are aware of the problem and will release a patch.

To cut this problem down to a minimum, I used CR Pro. and changed all my paths from relative to full. I now only occasionally get an error message, but the 'Mat' or materials load fine in spite of it.

Most of my content are in external Runtimes in a separate Partition on my Hard Drive.

Adriaan Barel (a.k.a. Vege Mite)
"Morality, like art, means drawing a line someplace." -- Oscar Wilde