To filter netwrok traffic, AdGuard uses a network driver. There are two options: TDI driver and WFP driver. While WFP driver is generally preferrable and is enabled by default for all newer Windows OS versions (Windows 8 and newer), it can potentially cause compatibility problems, especially with some antiviruses. These problems and subsequent errors can be very different in each case.
If you encounter a problem that you suspect might be caused by this, you can always switch to the older but more stable TDI network driver. To do so:
Go to Settings → Network.
Disable WFP driver as it’s done in this picture: