First, you need to clarify what you did with the trustee.
Even if you have a designated Roth account in your 401k, employer contributions (match) are pre-tax and must be held in a separate pre-tax account. If you do a direct trustee transfer, the Roth IRA trustee should refuse to accept a transfer from a pre-tax account, and should instead transfer that money into a traditional pre-tax IRA for you. Alternatively, if you intentionally put the pre-tax money into a Roth IRA, that would be a conversion, and is fully taxable in the year it happened. You will get a 1099-R and must report the conversion on your tax return and pay income tax.
If you did an indirect rollover, where you got a check from plan A and deposited it yourself in plan B, then you could have made this mistake. You should have gotten separate checks from the pre-tax and Roth 401k accounts. If you deposited them both into the Roth IRA, then one was a rollover and one was a conversion. You also violated the once-per-year limit on indirect rollovers.
So did you accidentally do a conversion? Were these direct transfers or indirect? What date did this occur? (In 2024? More or less than 60 days ago?)
At worst, you have a taxable conversion, but a conversion is not a contribution, so you can still make contributions. But you need to make sure this is reported correctly by bank B. If you don't want to do a taxable conversion, then you may be able to reverse the transaction, but it depends on the timing.
@dmertz
