Try the following:

1. Locate your Seal Online system folder (C:\Program Files (x86)\SealOnline Blades of Destiny)

2. Locate the file named "version"

3. Open the file and you will see something like this :
[DEFAULT]
CUR_VERSION=XRX8fXbnbkw=

4. change the text to:
[DEFAULT]
CUR_VERSION=XleDUuukBFg=

5. Then save and launch your Seal Online client as usual.