Most probably its the air suspension, better to change both. Did you have any messages from the iDrive screen?
Any of the site sponsor will be capable of changing the suspension.
To maintain warranty or if $$ not an issue, go back MA or PML, probably about S$10K+ for a pair. It would prob be cheaper if at other site sponsor.
To save $$, you can buy new set from Eng Soon for about S$2k+ each side and pass it to your fav workshop or site sponsor, prob about S$1k labour. Ask around for quote, wallet and time takes precedence.