Hi James,
You can't implicitly convert a vector to a scalar. In this case, the error is here:
float prevLum = texture(uPrevLuminanceTex, vec2(0.5));
Change it to:
float prevLum = texture(uPrevLuminanceTex, vec2(0.5)).r;
This is similar to the issue from the previous shader you posted. In general, the format of the message (ERROR: 0:30: error(#160) Cannot convert from: "4-component vector of vec4" to: "highp float") tells you two things: first, where the error is and second, what happened. The location of the error is first - 0:30 tells you that it's in the first shader string at line 30. Second, it says that you can't convert a 4-component vector to a float (I'm not sure why it says "of vec4", it should say "of float" - I will check on that).
It's likely that we were lenient in the past and allowed this to compile. Recent tightening of our rules may have broken your shaders. You should check on them systematically. If you run into harder issues, I'll arrange to take a look at your application.
Cheers,
Graham