Line 1 column 91 simply means the 91st character of the first line of the message.
In the first line of the received SIP message, there should be a "null" (most likely whitespace) at column 91, the 91st character of the message. If the errored line isn't shown in the event log, "debug sip stack messages" will show it. This can get noisy on a busy box, you may need to capture the output to parse later if you can't replicate it.
The TA900 isn't happy with the syntax of the SIP message as received and is flagging you that there was an incoming SIP message that it didn't understand and therefore didn't process. You'll need to work with the other end (which can mean arguing over RFCs and the nuances of SIP depending on circumstances) to get it straightened out.
Also check release notes for more recent firmware on the TA900 to see if this is a bug that was fixed. You may need to open a case with Adtran support if you can't resolve it with the sending end, for example if they insist that their SIP messages are correctly formatted.
I believe I found a ticket that you worked on this issue through Technical Support at Adtran. Was this issue related to having two Diversion headers in the SIP message? In this scenario, R10.5.2 firmware is required. Were you able to upgrade the unit to resolve the issue? If not, please let us know within this post or through your Technical Support ticket so that we can help you get this resolved.
Yes, the firmware update did solve the parser error issue. It did not fix the overall issue but it has been determined to not be an Adtran issue. Thanks for the help!