Well, first It is very unsual to offset on an account that is only 13 days past due. Off setting is usually a last resort type of collection method, and is not something that is done (or should be done) just because the loan payment is a few days late.
A bank can offset on accounts in the bank. Check the loan documentation and see if there is anything that relates to giving the bank the authority to offset against any and all assets the bank is holding in your behalf.
You should also call the "Loan Administrator or Senior Loan Officer of the bank. No sense talking to the loan officer as they will just try and justify what they did.
Ask them what authority they have to charge your "non loan related business account" for a personal loan payment, and express your displeasure with such.
The bank values your deposit accounts and your good loan history. If you do not receive a satisfactory explanation, or experience a good attitude or service by the bank in how they handle this, consider moving your accounts elsewhere.
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