Several Galaxy S8 and S8+ users have taken to Samsung and XDA forums for reporting issues with Qi certified chargers, including some of the older ones made by Samsung. Some of the known issues with the Qi chargers include the inability to use the Fast Charge feature, and at times failing to charge the device.
The affected devices are reportedly throwing up error notifications such as "Wireless Charging Paused" and "Irregular charging detected." Quite surprisingly, the newer Samsung chargers seem to be unaffected by this issue, while reports suggest several third-party brands with Qi-Charging capability are immensely affected.
Although there is no word on the root cause of this problem, it is confirmed to be widespread with inconsistencies in reports for different cables and Qi charger combinations, as reported by Android Police.
As a workaround, it is suggested that you reboot your device and that should fix the issue for a short period or until the next use of Qi-Fast Charging feature.