Hi Simon Ellis,
As you know that R12.6 is End Of Support. Hence it will be beyond the scope of support to address this issue.
Here is the background of the Error message:
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If a site turns on multi-tenancy while there are still untenanted requests and incidents in the database, they may receive large numbers of messages of the form:
AHD03114 Tenant for *** was not supplied and no default value is available
~Vinod.