iirc this might happen due to the attachments folder on server that is not yet created.
Do you know if the same scenario also fails on iOS?

yes this happened to me also and the fix options i did was the same as @woolfg mentioned

@Ivansss @SystemKeeper is it the same behavior on iOS?

We check the status code for the missing attachment folder and create it in that case:

https://github.com/nextcloud/talk-ios/blob/52a6f3de3c6859f2c8fc7d2cf5cda3f0da63d896/NextcloudTalk/NCAPIController.m#L1930-L1952

https://github.com/nextcloud/talk-ios/blob/52a6f3de3c6859f2c8fc7d2cf5cda3f0da63d896/ShareExtension/ShareConfirmationViewController.swift#L660-L668

is it the same behavior on iOS?

for me in worked on web and iOS as this was my workaround "Login to web or iOS and send a message/file in Talk there" to fix the problem.

The issue reported in my previous comment #4053 (comment) was unrelated with this issue.
This issue might be related with the attachment folder as @SystemKeeper reported here:
#4053 (comment)

TODO:

• try upload
• if it fails, use DAV propfind to check if folder exists
• if it doesn't exist (404 or 409), create folder with mkCol
  also see:

  • talk-android/app/src/main/java/com/nextcloud/talk/upload/chunked/ChunkedFileUploader.kt

    Line 98 in 9b3a115

    private fun createFolder(davResource: DavResource) {

  • https://docs.nextcloud.com/server/latest/developer_manual/client_apis/WebDAV/basic.html#creating-folders-rfc4918