Topic: Legacy Avisynth 2.5 is not supported, please use Avisynth 2.6

Starting from SVPflow ver.132 (SVP Manager ver. the legacy Avisynth 2.5 interface is not supported any more.

This means you'll see that error message in case the video player is using Avisynth 2.5 dll:

Legacy Avisynth 2.5 is not supported, please use either Avisynth 2.6 or Avisynth+

What to do?

Open "Information" -> "Events log" and find out the path of the wrong avisynth.dll.

The most common case is when you also have SVP 3 installed and you're using MPC-HC bundled with SVP 3:

23:25:20.415 [i]: Avisynth:, AvisynthMT 2.5, C:\Program Files (x86)\SVP\MPC-HC\avisynth.dll
23:25:20.415 [E]: VideoPlayer: Avisynth 2.5 isn't supported

Now you've three choices:
1. delete this 'avisynth.dll' and use the system-wide one from C:\Windows\SysWOW64
2. replace this .dll it with any other version you like
3. stop using MPC-HC installed into SVP 3 folder (or just use exclusively with SVP 3)