Investment complaints about Cofunds and Aviva to the Financial Ombudsman Service soared during the second half of 2018 as both companies struggled with replatforming exercises.
According to the Fos, 169 investment-related complaints were launched about Cofunds between July and December - with 63 per cent of these being upheld.
This is the first time in the past two years that Cofunds has even featured in the Fos data, which only lists companies which have had more than 30 complaints resolved.
Cofunds received the second highest number of investment-related complaints after Barclays, which received 324.
Meanwhile Aviva received the fifth largest number of complaints - at 144 - but a mere 25 per cent of these were upheld.
Aviva also saw a surge in investment-related complaints, having only received 97 in the previous six months and 89 in the six months before that.
Both companies have struggled to get service levels back to normal after replatforming during 2018.
Aegon, which bought Cofunds for £140m in 2016, merged it with its in-house platform over the May Bank Holiday weekend last year.
Since the replatforming was undertaken at the start of May 2018, Aegon's platform has been beset by a range of problems, including long waiting times on the phone and advisers struggling to get income.
Last month Aegon's chief executive Adrian Grace said its platform's service had returned to "target levels".
Meanwhile Aviva's platform was unavailable for six days after it moved to tech company FNZ in January, and there were ongoing issues with the new client reporting function and problems which affected payments for people in drawdown, while advisers said they were not getting their payments through the platform.
A spokesman for Aegon said: "There was a huge emphasis placed on restoring service levels associated with the Aegon Platform following the migration of former Cofunds users to it in May.
"Between July and December significant strides were made and resource mobilised to address service issues. By the end of the year core operational services had returned to target levels.
"The focus now for the Aegon Platform is on ensuring we provide the enhancements that advisers have requested. We are clear on the functionality that needs to be prioritised for the coming months and beyond this have a continual programme of improvements planned for the service."
Aviva stated there were two reasons behind the rise in complaints: the replatforming and Fos referrals on all Friends Life policies being included in Aviva numbers for the first time.
A spokesperson said: "There was an increase in the number of complaints received due to issues encountered on the adviser platform following our replatforming.
"We try very hard to resolve complaints before they are referred to Fos. We support the service Fos provides and work closely to ensure our customers get a fair outcome.
"The number of Aviva complaints upheld is below industry average for investments, which is a reflection of the efforts we go to to resolve complaints fairly before this stage is reached."