"Invalid Form" on SSP multiauth interface

Summary

We have been investigating the problems for several of the IdPs registered by our federation (18 out of 129 are in red), and in many cases, the error being detected is "Invalid Form”, but this does not seem to be the case (for instance, this one: https://sp-demo.idem.garr.it/Shibboleth.sso/Login?entityID=https://www.rediris.es/sir/uhuidp

Steps to reproduce

Search the entity https://www.rediris.es/sir/uhuidp on ECCS and retry the check.

What is the current bug behavior?

"Invalid Form" ECCS result.

What is the expected correct behavior?

"OK" ECCS result.

Relevant logs and/or screenshots

No.

Possible fixes

Check the form.