Here's one I've never run into before.
I have a Synology NAS drive that has worked flawlessly for 2 years. I use it primarily to share movies with my family using Synology app DS Video.
Yesterday I went to watch a movie on my iPad and received a message I didn't have permission to access. I rebooted both my cable modem and router and tried again - same result. Then I powered down my Synology - same result. My iPad is set using an external, fixed Ip.
If I set my DS Video login to the internal Ip, it works fine. I had my daughter try it from her house with the same app and it worked fine. I then took my iPad to a friends house and logged into the DS Video on my Synology successfully as I've always done.
So the bottom line suggests a problem with my wifi. I draw that conclusion because I only get the login failure when using the external Ip in my own network. As I said previously, the external Ip works fine when not connected to my own wifi. The problem with this theory is, however, except for attempting to login to my Synology box, my wifi works fine for everything else i.e. surf the web, receive mail, etc.
I'm at a point now of no ideas left so I'm turning to the experts. I hope someone has a clue what's going on.