It has been reported by the CIOT and elsewhere in the tax press that HMRC have identified software problems in relation to losses incurred by companies, “where the loss amounts reported in the CT600 form are different from the accompanying tax computations”.
HMRC have written both to the companies affected and to their agents.
https://www.tax.org.uk/hmrc-letter-differences-in-the-loss-position-of-company-tax-returns