Error: Acct: Invalid STOP record. [] STOP record but zero session length?" seems to indicate that there was a problem with the way the Accounting STOP record was formatted or the information ICRadius received may have been corrupted somehow. Perhaps there is an incompatibility with the Max TNT and ICRadius Accounting. What compatibility mode is your TNT configured for?
But seeing as this is accounting and not authentication, I do not see how this could be responsible for not allowing users to log in. In fact the STOP record would seem to indicate that the user did in fact log in. Otherwise you wouldn't even see a START record hence you would not get a STOP record. Therefore if users cannot login, it must be something else. You may want to run syslog on the TNT to an external syslog server to capture session information. Look for disconnect cause and progress codes to help determine why callers are getting dropped.