Save Content block changes results in error in getInstance()

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Hi,
I'm developing a site under C5.7.5.2 and keep getting problems when saving Content blocks. After a delay, I get:

Exception Occurred: ..path../concrete/controllers/dialog/block/edit.php:46 Call to a member function getInstance() on a non-object (1)

It seems that if the text in the block is only a few lines then all is OK, but beyond that I get the error. I'm not yet sure, but believe that this is also just intermittent too, so I'm guessing it's something server-related, but have no idea what.

I have PHP configured to allow up to 5MB posts and to use up to 256MB memory. Any ideas?