Comment to Pages: Content Box Character Limit?
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Got the answer. May help others at some point.
It was a ModSecurity issue. ModSecurity: Access denied with code 500 (phase 2) .
A global whitelist was created for my server ("set up a global whitelist for it, so rules 300015 and 300016 (they should both be generic SQL injection rules) should not affect anything at the /studio/ URI ")