Aegon’s integration of the Cofunds platform it bought for £140m in 2016 left many clients facing a litany of problems, including not receiving income payments on time, and with waiting times to contact the company.
Mr Grace said: "I know this is an important significant issue for advisers, including financially. But for Aegon UK this is not a material amount, not a significant cost.
"We are sorry for what is happening and we fully accept there have been issues with it. Advisers should appreciate we are a very big business and Aegon is part of a broader group."
The company said its standard complaints and compensation process would apply.
Aegon said the replatforming cost it £3m more than expected in the first half of the year. Mr Grace said he expected the bill for the rest of the year to be higher than £3m "but not in the tens of millions".
He said: "Knowing what we know now, I can say we would still have gone ahead with the replatforming, but we would have put more contingency plans in.
"We tested the platform extensively, but there should have been more contingency. Our call volumes went from the normal level of 2,500 to 9,000. Our worst expectations were overridden."
Mr Grace said he "hasn’t thought much" about plans for the business for the rest of the year, "as we want to make the foundations are right before we try to build anything on them".
The replatforming happened in May, but advisers have continued to suffer and Ian Lowes, who runs Lowes Financial Management in Newcastle and is an Aegon client said he was "holding out for improvements", but that the problems have "cost us dearly".
Jenny Griffiths, of CP Griffiths, an advice practice in Stourbidge, said she had not noticed any improvement in the service, except that the phones were answered more quickly.
She said the issue could cost her as much as 10 per cent of the turnover of her firm, as he transferred clients away from the platform, meaning they were out of the market and couldn't be billed.
She said she had sent invoices to Aegon for payment for time she has spent on the phone to the company, and was told under the complaints procedure it takes six working days for a response, and while she has received an initial response, she has had no clarity on the next steps.
Ms Griffiths also expressed scepticism she would be able to put a proper monetary value on all of the ways it had cost her business.
Ms Griffiths is endeavouring to transfer her clients away from Aegon, and she said: "I can’t wait to get my clients off the platform. There are just so many problems. I had a client who wanted to pay by direct debit, so we set it up to start in July, but the direct debit didn’t happen, there seem to be a lot of these glitches."
Mr Grace said the Aegon platform business didn’t really exist when he took the helm nine years ago, but he now expected the future growth of the business to come from the platform, rather than the traditional life and insurance business.
He said the life and insurance arm of Aegon was "an old business, running out of steam".
Mr Grace added: "It will still be there for 20 or 25 years, but the growth will come from the platform."
Sam Holder, operations director at Chelsea Financial Services, which uses the Aegon platform, said: "Cofunds' migration to Aegon has not been without its teething problems but, in our view, things are starting to get back on track.
"Functionality has improved in some areas compared to the original Cofunds platform. For instance, clients can now switch their contributions from a GIA to an Isa online, as well as withdraw money from their Isa online.
"Aegon itself admits there is still some way to go to in terms of improving its level of service. But at the same time, we think the migration has been unfairly compared to incidents such as the IT upgrade debacle at TSB in April this year.
"Aegon has moved all of its clients' assets successfully; there were some difficulties regarding getting access to portfolios due to the sheer volume of traffic, but this has since been dealt with.
"Our staff have been working longer hours and over weekends, to ensure that any clients requiring guidance could get the help they needed. So, in that sense, we did incur extra staff costs.
"There were some backlogs with certain client transactions and Aegon is now working on a price comparison for those affected."
david.thorpe@ft.com
