Here are some common causes for internal server errors when adding Instagram accounts (if you're getting the internal server error elsewhere, this article won't help).
(1) You haven't ticked all of the boxes
We need you to check all of the boxes in the 'add account' window to say that your account meets our criteria:
(2) You have entered your username with a different symbol, or an email address
Make sure you just enter the username of the Instagram account, not the email address or the username including the @ sign. For example, if I was adding the account "nike" I would just enter "nike", not "@nike" or "email@example.com".
(3) You are on our old billing plans and have reached your billing limit
If you have been a Schedugram customer since the early days (2013/14) then you might be on our old billing plans, where you pay for a certain maximum number of accounts. If that's the case, you might get an error when adding new accounts as you have reached your limit. Reach out to support and we'll change your plan.
If you're not sure what kind of billing you're on, head to the "Billing / Profile settings" page and click "Subscription" on the right - you'll see a 'currency and plans' section. That section will say what type of billing plan you are on.