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

I have had this happen to me a lot and what usually fixes it for me is to close the game and reopen it, also you can let it finished with the playback speed set at 1x and see if it goes from gray to black.

If that does not work try going to another replay and see if it does the same thing, I have found sometimes that fixes it. I am not sure what else to try other than that. Usually this thing fixes itself though.

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.

/opt replaycache 1
Might help

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