Enabling the collabora v3.2.0 on NC15 fills the log with the following errors on every file operation:
fclose(): supplied resource is not a valid stream resource at /var/www/nextcloud/apps/files_external/3rdparty/icewind/streams/src/Wrapper.php#132
_StreamDecorator::_toString exception: RuntimeException: Unable to seek to stream position 0 with whence 0 in /var/www/nextcloud/3rdparty/guzzlehttp/psr7/src/Stream.php:200 Stack trace: #0
These errors appear on every file operation, not only on opening office files. Apart from these errors, collabora is working flawlessly, can open/edit/save all files.