Not been but I'll have a look.
I just went to check and found I can't do anything in the nether either - which is different to how it was
Not checked yet, want me to just restart the minecraft server and give it a go then?
Server restarted. Give the nether a go.
Give it a nether go, surely?
Never mind then. I shall consider other lighting solutions!
Have you turned it off and on again?
When did it start doing this?
When was the last time it definitely worked successfully?
What mods/config were changed during that time?
Is there anything unusual looking in the logs?
</debugging-for-beginners>
:P