WebLink 9.0 does require Laserfiche Server version 9.1 or higher. Laserfiche client is entirely separate from WebLink, and you can have any combination of the two products installed on a given machine. That is, reverting the client install isn't necessary and won't resolve this issue.
For your information, the full Product Compatibility List is available here:
https://support.laserfiche.com/ow.aspx?Product%5FCompatibility%5FList
Feel free to ask if you have more questions.
Also, you can upgrade the Laserfiche service to 9.1 or higher without changing/upgrading the clients. This would allow you install WebLink 9 and keep using LF client 9.0.