Block can't be saved because of the form in it

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This is the second time I encounter this strange behavior. I have a form on my website which has an action="something.html" attribute.

When editing the block that contains that form, C5 won't let me save it. On the previous website I changed the form to one that doesn't use action attribute and it worked. I know I could do it again, but I am really curious why does C5 won't let me save this block?

What is in the action attribute that interferes with C5 and doesn't allow saving?

Madebym