The cause looks like due to Safari 12.0. Changing browser to Firefox 57 seems resolve the issue. Sorry!
Procedure to reproduce
Clean install OctoberCMS 437 and RjGallery 1.2.2 Plugin
Create new gallery named "ABC DEF" slug "abc-def"
Upload 30+ images in total of 60 MB
Click [Save and Close]
Click on gallary "ABC DEF"
Try to upload one more image
Cannot upload any file, just results broken file. Even in "Media" tab of OctoberCMS once this happened.
If I uploaded the same files to "Media" prior to upload RjGallery, this never happens.
After this happens, I cannot upload from backend but I can create 100MB+ files in storage/app/ from console.
The free space of the partition is more than 190GB+.