This type of error might occur if something was wrong server side. In case, please send us a report with last data sent to server and last steps you did before this happen.
The HTTP method used is not allowed for the API method. For example, you tried to retrieve data with a POST action on an API method that only allow the use of GET.
The format of the date/time field you provided could not be validated as ISO format. The standard ISO format is "YYYY-MM-DD HH:MM:SS", for example "2011-01-15 11:59:59". In some API methods, you can use "NOW" to point to the current date/time.
IP format could not be validated. Private (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and reserved (0.0.0.0/8, 169.254.0.0/16, 192.0.2.0/24, 188.8.131.52/4) ranges are not allowed either. An example of a valid one could be "184.108.40.206".
Only one import can be running at the same time.
Campaign already scheduled.
Scheduled datetime not found.
Invalid ABSPLIT parameters.
Invalid ABSPLIT subject B.
Invalid ABSPLIT send_after_period.
Limit parameter is bigger than expected.
The offset passed is invalid. Range is from 1 to 50.
Show ID is not present or it does not belong to the current user.