The OO Connector is not yet released for Nextcloud Server v32 (which itself is not yet released outside of early betas). I presume you force enabled it.
The OnlyOffice folks know about some of the issues based on what I see in GitHub. Based on a quick look I would expect problems trying to use it with the early betas of v32 for now.
There are likely errors in your server-side log (which can be checked by following the instructions in the support template).