I have encountered this same message & problem in Designer.
The file did have 5 art boards to start, then modified to 3, saved to local hard drive. (Cloud backup every 4 hours, so the backup file is the same)
I printed the file for client to approve, make notes, and hadn't opened the file in "about" 2, 2.5 weeks awaiting their feedback.
I cannot remember exactly how I saved it at the time. (cntl+S or through tool bar menu). Probably ctrl+s, coming from Adobe, that is very habitual now.
Designer installed on Dell Alienware 17, Win10 pro 64bit, 32GB RAM, 1.5TB HD, Intel i7-4900MQ CPU @ 2.80GHz NVIDIA Geforce GTX 780M 4GB.
I can send the file (253MB) if you are interested in deconstruction.