Errors for topic pages only

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On my site - www.farmingfornature.ie/forum/ - the forum appears fine. But when I click on links to topics - like www.farmingfornature.ie/forum/grassland/ -  I get a 500 error. It was previously working. I've disabled all plugins, updated everything this morning - including to ForumWP Pro, but it still isn't working. Any ideas?