An error occurred during the processing of the page. The server block is not well formed.

Reason:

Custom page layout/aspx page has some tags that were registered in the custom master page. So, when the custom page layout looking for 'registered tags' in the default master page.. failing to bind.
Precisely, Actual custom masterpage is not binded to the custom page layout due to the late binding.

Solution:
Right click on the actual master page in the SPD and select 'set as custom master page'.