You cannot share and upload photos from Gallery with Galaxy S20+. However, you will not be able to upload it after updating Next Cloud someday. Unable to upload file since last Samsung update. Is there a solution to this?
I don’t think that the problem is related to the latest Samsung update, but the latest Androit app update, which you would have found-out if you had tried searching for an answer first before opening a new thread 
I have the same issue since 2-3 weeks on a Huawei P30, Andriod 10, EMUI 10.1 with Nextcloud app version 3.14.1
Quite annoying, because the upload/share is my favorite function of Nextcloud.