Don't wont to start new topic. After upgrading from 2.6.12 to 2.6.13 I'm getting this error massage. After clicking OK anything seems to be working fine.
My system is Win7 64-bit and Avidemux is also in 64-bit version.
Simplest is to deinstall fully and reinstall, there is an old plugin somewhere
Full reinstall worked as suggested. Thanks.