What version of Sage CRM are you using?

CRM 2017 R2  From Sage 2018 installation

Hiya

'Solo' was a feature that was discontinued in Sage CRM 7.2.   Has your instance of Sage CRM been upgraded from a version before Sage CRM 7.2?

I have not seen the error before but the good news is that the Logs (SQL and System Logs) should show very clearly what has gone wrong.  You partner and the local Sage team will be able to diagnose the issue.

Probably a TableScript on the company entity

Administration > Customisation > Company > TableScripts

Can you see anything containing the following? 

var sIDCust = Comp_IDCust;
if ((sIDCust != null) && (Defined(sIDCust)))
{
if (!IsSOLOClient())
ErrorStr = CustomerUpdate('company');
else
if (AccpacDataChanged())
ErrorStr = CRM.GetTrans('Accpac_Messages', 'NoAccessOnSOLOClient');
}

If so..

It may need your partner to take a look