[TYPO3] Reference Index Update after upgrading from 3.8.1 to 4.0.1

["Jesús H."]

Dear list,

I am having a problem after upgrading from 3.8.1 to 4.0.1. I am getting 
errors on the FE and BE listing the page tree.

When I try to update the Reference Index I get the following:

Warning: Invalid argument supplied for foreach() in 
/home/httpd/vhosts/domain.com/httpdocs/wfa/t3lib/class.t3lib_refindex.php 
on line 656

And it doesn't updates and/or check the "test reference index".

Does anybody have a clue of what happens?

With best regards,

Jesús H.