New Monitor = Black Screens on startup of Mac Mini 2018

Hello all, I wonder if you know anything about this type of thing. tl;dr: I bought a second hand monitor, which works fine but prevents my system from booting up properly. The new monitor is one of three. If it's plugged in, everything's just black upon powering the Mini. If I unplug the new monitor, all boots up as normal. I can then plug the monitor in and carry on as nomral but it's a big pain getting round the back of my desk each time. Would love to solve this! I think I have a bit of trouble with the power bus, some times peripherals don't power on and I need to unplug another to make it work. Wondering if it's something to do with that. THANKS! Here's my full spec: # Hardware Overview: Model Name: Mac mini Model Identifier: Macmini8,1 Processor Name: 6-Core Intel Core i7 Processor Speed: 3.2 GHz Number of Processors: 1 Total Number of Cores: 6 L2 Cache (per Core): 256 KB L3 Cache: 12 MB Hyper-Threading Technology: Enabled Memory: 64 GB System Firmware Version: 2020.0.1.0.0 (iBridge: 21.16.365.0.0,0) OS Loader Version: 540.120.3\~37 # Graphics/Displays: Intel UHD Graphics 630: Chipset Model: Intel UHD Graphics 630 Type: GPU Bus: Built-In VRAM (Dynamic, Max): 1536 MB Vendor: Intel Device ID: 0x3e9b Revision ID: 0x0000 Metal Family: Supported, Metal GPUFamily macOS 2 Displays: BenQ GL2480 - Via VGA/Thunderbolt adaptor: Resolution: 1920x1080 (1080p FHD - Full High Definition) UI Looks like: 1920 x 1080 @ 60.00Hz Framebuffer Depth: 30-Bit Colour (ARGB2101010) Display Serial Number: J1N03729019 Mirror: Off Online: Yes Rotation: 180 Connection Type: Thunderbolt/DisplayPort DELL 2407WFP - Via VGA/Thunderbolt adaptor: Resolution: 1920x1200 (WUXGA - Widescreen Ultra eXtended Graphics Array) UI Looks like: 1920 x 1200 @ 60.00Hz Framebuffer Depth: 30-Bit Colour (ARGB2101010) Display Serial Number: CC30369Q003S Mirror: Off Online: Yes Rotation: Supported Connection Type: Thunderbolt/DisplayPort PHL 322E1: Resolution: 1920x1080 (1080p FHD - Full High Definition) UI Looks like: 1920 x 1080 @ 60.00Hz Framebuffer Depth: 30-Bit Colour (ARGB2101010) Main Display: Yes Mirror: Off Online: Yes Rotation: Supported Adapter Type: DVI or HDMI Adapter Firmware Version: 2.19