Same issue here with multiple Huawei phones (e.g., P30 Pro) after an update around May (also fails on paid unlocked Android app).
There is an inconvenient workaround: Instead of saving (which fails with “Failed to save image”), “Share” the picture and use a file explorer action to “Save as …” locally. Sharing will always use the same hardcoded filename “image.JPG”, so a file explorer that supports automatic renaming (instead of failing on trying to overwrite an existing file) is convenient when sharing multiple pictures. File Manager+ supports automatic renaming. For single file exports, the open-source file explorer Amaze File Manager works well.
Nevertheless, a fix for this issue is overdue given that downloading a picture is a core functionality of Plex photo albums. This bug should be prioritized, especially now with new Plex users migrating from Google Photos (it appears this issue spreads across several other discussions; e.g., Failed to Download Image Error all the sudden).
