Problem with 5.51 and 6 and Mercalli 1 & 2

[volki]

Using clips made with E-5.51 & Mercalli-1 and imported in E-6.01, the clip will show after about 1 /3 the Mercalli Text
In E-5.51 the finished clip (HDV) is rendered and exported in HQ fine. Then reimported in another system, with E-6.01 and Mercalli-2
When this clip is on the timeline, it plays ok at the beginning, but after about 1/3 it will go black and show the Mercalli-1? text

Any ideas what’s happening?