This seems to be a problem with a limited number of devices. I'd like to gather some information from those of you seeing this though!
Things that will help the most would be the exact model number and Baseband version (both under Settings>About Device) for your device as well as your carrier, when you started noticing this happen, and what you've already tried to fix it. Thanks!
I starting experiencing this same problem starting on 7/10/17.
I had the same charging problem using the supplied charger and cable. The charging port on the phone proved to be the problem. I exchanged the S8 for another and the cable fit much tighter. Looks like a batch was defective.
Likewise, you can always reach out to your carriers for an exchange like @userKMCLZoHoK4 mentioned.
My daughter's S8 began having this exact problem. She leaves it on the Samsung original equipment charger all night. It only reaches 5% charge. I was going to buy a Samsung Galaxy S8+ next week, but now I am reconsidering. How is Samsung going to fix the S8 and S8+ phones? Last week, I had a Galaxy S7 Edge blow up. It caught on fire and smoked for many minutes. I've always thought that Samsung phones were the best, but these issues are becoming a big problem.
I purchased a bag that comes with a joey charger to use with mobile phones and got the incomplete connection error message. I called your number and was told that you can only charge your phone with a power source from Samsung. That's pretty manipulative and lame. Why should this be any different from plugging into the wall or a computer to charge???
I called again and the person i spoke with this time said that you can charge with other sources as long as you use the Samsung cord and to contact the company that made the power bank...
I am having the same problem; I had to return my phone after 2 months they sent me a new one which worked for a while but now I am getting the same incomplete connection message. This is super frustrating Samsung has gone wayyyyy downhill with the S8!
After two weeks of no resolution, I called Samsung and they wanted me to rest my phone. I didn't want to go through that process, so I took my device to a Verizon store. We plugged the device into multiple chargers with no success. Verizon recommended I replace the phone.
Thinking the port is dirty, I went to clean it one more time. I used compressed air and this time added suction from a vacuum. I plugged the device back into a USB-C charger and it started working as normal. I was surprised.
My assumption is there were dust or sand particles that were causing the problem.