Hi Omar and Giuseppe ,
Finally we found two problems that were causing troubles.
One problem related to switch between companies was resolved and was related to some configuration on Global users and a tick in Optional Functionalities.
But the other error is some kind of bug similar to the error specified by Neal.
We add a new field to standard Block "TaxAuthIDBlock" and that was cousing troubles.
That block is only use in Bolivia and when users tried to login into the company with that VAT Zone, error appears.
Since the block in 8.0 Register format, must be defined again with all standard fields and new ones, I think there is some problem how SERP is creating database on Client Side. I don´t think SERP is taking notice of that new field and then then block structure between Client and Server is different.
I delete the new field and everything was Ok. Also the hal file with the Updated window definition to show the new field, did not provoque hal error. BUT when user tried to login into second (Bolivia) database, the error appears in server log, but nothings happends.
I have other clients with modified standard Blocks but this is the only with with multiple companies with different VAT Zones.
Regards