I'm using magento ver 220.127.116.11 and I'm getting net::ERR_EMPTY_RESPONSE error while updating or adding products for certain admin created custom attributes.
Tried with reindex and cache flush but went in vain.
Complete URL looks like this - admin/catalog_product/validate/id/3488/back/…o_tabs_group_494/store/0/key/6f96bad4df91313955054abdcd012021/?isAjax=true net::ERR_EMPTY_RESPONSE
I'm pretty sure you can find more information from var/log folder. Also, it's good idea to wipe var/session folder after upgrade.