I had exactly the same problem in latest Xubuntu LTS, and all I had to do to bring back control was to turn off checkbox in Power Manager with label "Let Power Manager control screen brightness" (or something like that). Unfortunately, I cannot see such feature in Linux Mint. Any advice?
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System: Host: ThinkPad-SL300 Kernel: 4.4.0-53-generic x86_64 (64 bit gcc: 5.4.0)
Desktop: Cinnamon 3.2.7 (Gtk 3.18.9-1ubuntu3.3) dm: mdm Distro: Linux Mint 18.1 Serena
Machine: System: LENOVO (portable) product: 2738B4G v: ThinkPad SL
Mobo: LENOVO model: 2738B4G v: LENOVO 6AET64WW Bios: LENOVO v: 6AET64WW date: 12/01/2010
CPU: Dual core Intel Core2 Duo P9700 (-MCP-) cache: 6144 KB
flags: (lm nx sse sse2 sse3 sse4_1 ssse3 vmx) bmips: 11172
clock speeds: min/max: 800/2801 MHz 1: 2133 MHz 2: 2133 MHz
Graphics: Card: Intel Mobile 4 Series Integrated Graphics Controller bus-ID: 00:02.0 chip-ID: 8086:2a42
Display Server: X.Org 1.18.4 drivers: intel (unloaded: fbdev,vesa)
Resolution: 1280x800@60.00hz
GLX Renderer: Mesa DRI Mobile Intel GM45 Express
GLX Version: 2.1 Mesa 11.2.0 Direct Rendering: Yes