"" is not a valid integer value

This happens because there is a company number in cxClient.InternalClientId that is not a valid number.
It must be resolved via SQL.

Run this:

select distinct case when charindex(';', InternalClientId) > 0 
    then substring(InternalClientId, 1, charindex(';', InternalClientId) - 1) 
    else InternalClientId end
from cxClient

Check that one of the rows has a NULL value.

Then run:

select * from cxclient

Find the row where InternalClientId is NULL.
Run delete on the row with the NULL value in InternalClientID:

delete cxclient where ClientId='clientnumber'

Example:

delete cxclient where ClientId='16'