Had this happen twice over the years.
On both occasions it was the power supply. The power supplies were no longer capable of providing enough current, despite the lights illuminating on the box.
Nowadays power supplies are 'switch mode'. Instead of using a transformer the correct voltage/current are created electronically. Although usually they just fail completely with age, they can apparently be working, but the current they put out drops.
It took me ages to work out what was wrong originally. As the lights on the Slingbox came on (except the Network) I assumed the power supply was OK.
Eventually it was the last thing to try. As soon as I connected a new supply everything worked.
The Slingbox power supplies are 5v at 2A (amps). From my experiences I think Sligboxes need to draw close to 2A to function correctly, so a slight current reduction in a 2A supply will cause the problem. Out of interest a few weeks ago one of my IP cameras started to fail. That too was caused by its switched mode power supply suddenly failing to supply enough current, although all the lights came on etc.
Having had two of Slingbox power supplies fail, I last replaced mine with a regulated 5v 4A supply, which are advertised as Slingbox compatible, on eBay. A 5v 3A one would be fine too. That extra bit of available current is useful. The Slingbox will only draw the amount of current it requires.
I have had a Slingbox Classic, Slingbox PRO, and PRO-HD. All used the same power supply same voltage, same current, same cable/plug. I assume your Solo will be the same, but I do not know for sure.
If you are electronically inclined, then any regulated 5v, DC, 3A to 4A supply would do. When I was testing my Slingbox, I initially used a spare Slingbox one. When that powered up everything correctly as a temporary measure I used a 3A variable power supply, set to 5v. I cut off the cable from the failing supply and used that. Obviously you need to ensure the polarity is correct (Plus and Minus), before connecting it up to the Slingbox, so a multimeter is essential to test that. If you are unsure / do not have the knowledge then definitely just go for a power supply that is advertised as Slingbox compatible.
I cannot guarantee that it is definitely the power supply of course, but from my experience that was the cause of exactly the same Slingbox problem you are experiencing on two occasions. Hopefully your Slingbox has not developed a fault, and this solution works for you to.