Dear All,
the problem is solved; it was caused by a SPAU adjustment after the upgrade.
The adjustment restored an implementation on CL_THTMLB_DROPDOWNLISTBOX IF_BSP_ELEMENT~DO_AT_BEGINNING due to the application of a SAP Note 0001844041 related to number of entries in the dropdown list.
By retrieving the change implemented by the upgrade, the problem was solved.
Thanks a lot
Francesca