Nordic investment portal Nordnet began working normally again just after 4pm on Tuesday afternoon following a service outage earlier in the day.
The service suffered a fault on Tuesday that allowed some users to see information associated with others' accounts.
The service was down for some time on Tuesday afternoon as a result.
The company's Finland country manager Suvi Tuppurainen told Yle that it was possible that users could have seen account information associated with other users' accounts, including stocks and investment funds held in their name.
She said that it was not, however, possible for people to transfer others' money into their own accounts.
The problem affected all four Nordnet markets: Finland, Sweden, Norway and Denmark. In Finland, the company has some 600,000 customers.
"There is a technical problem behind this, so according to the information we have at this moment this is not a cyber attack," Tuppurainen said.
The company is headquartered in Sweden, and it has reported the outage to the Swedish financial regulator as well as Finland's Financial Supervisory Authority.
Yle spoke to one Nordnet user who had logged in and saw other users' details, and had the impression they would have been able to buy and sell shares in the account.
"I'm really, deeply sorry for this," said Tuppurainen. "I promise that we will do everything we can to get clear log data on whose information has been visible to others. We will also do our best to ensure that this never happens again."
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EDITED on 11 February at 2.55pm to remove mentions of a cyberattack as the cause of the outage.
EDITED on 11 February at 4.40pm to update that the service was working again.