Help! 'The White Screen of Death' when adding new block

Try the suggestions described here.
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... Morpheus: What is "real"? How do you define "real"? If you 're talking about what you can feel, what you can smell, what you can taste and see, then "real" is simply electrical signals interpreted by your brain...

Blocks can be tricky. One erroneous line of PHP can cause the infamous white screen. Whenever I want to add a home-made block, I try the code first in a node before putting it in a block. When the node fires the white screen, that doesn't matter so much, because the rest of the site remains accessible. Another "wise advise" is to put the visibility of the block only to 1 particular page. If it goes wrong, the other pages/nodes would at least still be accessible.

But your block is already there, causing the white screen. If nothing works, the best thing to do is to go directly into your database with PHPMyAdmin or another similar tool, open the {blocks} table (or your_prefix_blocks), find the block which is causing the trouble, and replace the "1" in the Status column with a "0". That disables the wrongdoer.

That should do the trick. At least, it did so for me.