While you get this sorted out, you can disable the card-based checkout option in your cart by going to Seller Admin > Edit Profile, under Accept Credit Cards select "My payment processor will charge cards for me", then click Submit to apply changes.
It appears that PayPal has recently expanded the limits for those PayPal Manager credential fields at their end, but the corresponding fields at our end still have the original limits. The most expedient solution would be changing your Merchant Login to be 20 characters or less. If you need assistance to accomplish this, we would recommend contacting PayPal support at 888-221-1161; that number should get you to an elevated tier of tech support better acquainted with E-junkie than their regular front-line customer service agents.
Expanding any field sizes in our database is a major operation that locks up the database while the change is being applied, so it's not a simple change and must be planned for a time that would minimize the impact to our service. Development has been planning to upgrade our DB servers to SSDs (solid state drives) very soon, which are much faster than traditional hard disks, so completion of that upgrade is probably a prerequisite for any DB structural changes.