The replay playback option is greyed out.

When I press , or . the counter stays grey and the text changes (0.40x, 0.50x) yet the speed stays the same.

How can I fix this?

You need to let the replay finish playing or re-open it,

Doesn't work. Still grey

Where are you loading the replay from?

when you select "quick load" you can't set the speed, but if you click "load" you can.

When you select Quick Load, you can change the speed after the replay fully plays once - but Thugoid already specified that this didn't work.

I have had the same problem it seems to me that it just works when it wants to. I will note it down in our bug reports so that it gets addressed.

I've tried all of your suggestions but still nothing ;(
I'll see if it fixes itself