Checking the log file of Magento 1.9, in two web site, I find this row:
exception 'Mage_Core_Exception' with message 'Invalid block type: ES_Newssubscribers_Block_Newssubscribers
How to remove this type of error?
Whitout a stacktrace it's hard to say what the exact cause could be.
If the class
ES_Newssubscribers_Block_Newssubscribers exists in your code (custom module is active) one issue could be that the block doesn't inherit from
Mage_Core_Block_Template which extends the abstract block).
I would suggest to check if that block exists in your code and the module ES_Newssubscribers is active and if yes if it inherits from the above mentioned class.
To remove the error you will either have to make sure the block exists or find the corresponding layout definition and remove the block if you don't need it at all.