This error could be thrown as a result of the user change logging where a column is added to a xf_user_* table that has a default value of NULL.
As soon as the DataWriter changes that value, this error will be thrown by the change logging system when it attempts to write the NULL to the old_value field.
It can be worked around (the only cases where I see this being a problem in my add-ons can be solved actually by excluding these from the change logging) but it might be better to catch this in the code and convert it to an empty string or 0 or allow NULL for that field.
As soon as the DataWriter changes that value, this error will be thrown by the change logging system when it attempts to write the NULL to the old_value field.
It can be worked around (the only cases where I see this being a problem in my add-ons can be solved actually by excluding these from the change logging) but it might be better to catch this in the code and convert it to an empty string or 0 or allow NULL for that field.
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