I experienced this or a similar issue:
1) on iPhone #1, I could login with either of my two OBiTALK account credentials
2) on www.obitalk.com
I could login with either of my two OBiTALK account credentials
3) THEN I upgraded the firmware (to 1.2.1) of Obi110 #1 and changed the Device Display Name from 'iPhone' to 'iPhone 4' in OBiTALK account #1
4) I left the firmware of Obi110 #2 (at 1.2.0) as is (did not upgrade) and I changed the Device Display Name from 'softphone' to 'iPhone 3G' in OBiTALK account #2
5) NOW on www.obitalk.com
, I can login with either of my two OBiTALK account credentials
6) BUT on iPhone #1, I can login with OBiTALK account #2 account credentials, but not with OBiTALK account #1 account credentials - the error is 'Error Signing In' 'Check your User ID and Password'
7) I uninstalled the OBiON app on iPhone #1, and re-installed, but that did not solve the problem
Was it the firmware upgrade? Was it the Device Display Name change?