How to delete off-line filesā€™ cache in the iPhone app

My Nextcloud app on my IPhone takes about 15Gb. I guess it is due to massive off-line files pool. I do not see any option to clean off all those off-line folders and files at ones. Did I miss some features? The folder Manage offline files does not contain any files at all but I have marked many files ā€˜offline availableā€™. There is no way to clean the cache one-by-one. Please, help