Mail folders empty on Yahoo account
I have a Macbook air running on Yosemite 10.10.1 and an iPhone 5S running on iOS 8.1.2.
On both devices, since few days ago, some Yahoo! Mail subfolders do not show any messages, as if they were empty.
I have tried to delete the Yahoo! Mail account on both devices and to create it again, but it doesn't work, a couple of folders, that were running perfectly before are now empty.
Additionally, if I have an email in the Inbox and move it to one of the failing folders from any of the two devices, it appears to move correctly (actually it becomes the only displayed email in that folder on that device), but then it is not visible on the other device.
I have checked the Yahoo! Mail web page and all the folders appear to be running properly.
Can somebody help me to fix this problem?
MacBook Air, OS X Yosemite (10.10.1)