Cofunds has the most investment-related complaints currently in progress, according to the Financial Conduct Authority.
According to the latest complaints data from the FCA, published this morning (April 18), the platform reported 6,204 complaints during the second half of 2018.
Aviva Life Services and Prudential Assurance Company came next after they also received a large number of complaints in this category, at 4,546 and 3,756, respectively.
Cofunds and Aviva suffered prolonged servicing issues last year, after they changed their underlying technology provider. For Aviva problems started in January, while Cofunds started its ill-fated replatforming over the May bank holiday weekend.
In comparison, in the second half of 2017 Cofunds received 504 complaints, while Aviva reported 3,006.
In the first half of 2018 Cofunds had 2,333 open complaints about investments while Aviva had 3,772.
Meanwhile Phoenix Group’s subsidiary Abbey Life Assurance was the most complained about investment provider, with 17.9 per 1,000 client accounts.
And the Share Centre was the most complained about decumulation and pensions provider, with almost 13 complaints per 1,000 client accounts.
Aviva Life Services reported the most open complaints concerning decumulation and pensions during the period from July to December 2018, totalling 11,078, followed by Prudential Assurance Company at 7,439 complaints, and Royal London Mutual Insurance at 4,100.
However, overall complaints to providers fell during the second half of 2018 for the first time since the complaints reporting procedure was changed in 2016.
Prior to the end of June 2016, firms were not required by the regulator to submit complaints data for those resolved by the end of the following business day after they were initially received.
Since June 30, however, all complaints received by a company are deemed reportable by the FCA.
Christopher Woolard, executive director of strategy and competition said: "It is encouraging to see that complaint figures have dropped and firms are dealing with complaints more quickly.
"We expect firms to continue to focus on ensuring their customers are well served and that they respond quickly where consumers complain."
Total complaints in the year fell by 5 per cent from 4.13m to 3.91m.
Payment Protection Insurance claims made up 40 per cent of the complaints. Excluding these, complaints decreased from 2.41m in the first six months of 2018 to 2.33m in the latter half of the year.
Outside of PPI, current accounts were the second most complained about product, although these also decreased during the period by 13 per cent.
Credit card complaints, however, rose 10 per cent during the final six months of 2018.
The proportion of complaints closed by firms within three days rose to 37 per cent, compared with 35 per cent in the first half of 2018, while those closed within eight weeks rose from 92 per cent to 95 per cent.
Adrian Lowcock, head of personal investing at Willis Owen, said: "Whilst it is good to see the overall number of complaints to fall, it is clear that the financial services industry as a whole still has work to do.