Getting error submitting cart when activating Premium trial

[Aronia]

I got an Inspire HR about a year ago, but didn't get around to trying the Premium membership until now.

Yesterday I tried to register for a Premium membership but after I'd completed the checkout and authenticated the $0.00 payment I got a vague error message. Later it struck me that the card I used will expire before the free trial ends, and I suspect that was the problem.

Today I repeated the process with a recently activated card, and at an early stage a message flashed briefly on the screen saying only one membership can be active at the same time. But according to my Fitbit account settings I still didn't have a Premium membership so I went through with it anyway. This time, when I'd completed the checkout and authentication, the checkout page stayed greyed out and kept showing the spinning circle. It still does, an hour or so later.

Please look into this, and let me know what I should do!

Moderator Edit: Clarified subject

[Aronia]

Above I described my two first attempts to register for a Premium membership (both failed).

Today I tried for a third time, at https://www.fitbit.com/global/eu/checkout like before, and got the same error message at checkout as the first time:

"Error submitting cart: Error: Request failed with status code 500"

What can I do to work around this problem, and activate my free Premium trial?

[SilviaFitbit]

@Aronia Hi there. Thank you for the detailed information shared. 

I appreciate that you shared the specific error message that you received. I was able to see that you've already contacted our Support team. 

Have a nice day.