Dear Anothai,
I am not sure if this issue can be answered in a general way via this forum - the error TPM_TRAC1899 is a very general one, triggered within the Treasury Accounting Adaptor (TRAC) in case of unexpected situations such as inconsistencies.
So without checking directly why it is triggered, it is hard to tell how to overcome it.
Maybe you have a consultant onsite which could have a look at it in debugging mode.
As you state that you convert only OTCs, you could check if you can find out at least for which deal(s) the error message is raised. If these are 'old' deals (end of term in the past), you could consider to exclude them from migration via report RFTR_MIGRATION_EXCLUDE_OTC as a quick fix.
Best regards,
Marie