Just throwing this out there.. my understanding is that the EFI partition needs to be 512Mb, Would it be possible for you to make a backup of your EFI partition and then resize it to make it 512Mb rather than 400Mb?
Also, I had problems installing Solus on my PC. I had several storage drives, one dedicated Windows drive, and one dedicated Fedora drive. When I tried to install Solus on my Fedora drive, it failed. Despite having it's own separate drive, the Windows install was interfering with the Solus install. As soon as I disconnected the Windows drive it installed perfectly.
Before you try anything else, I highly advise you to try and make the EFI partition 512Mb. However, since Solus 4 has come out, I've fully left Windows 10 completely (was to scared to do so before) and I am leaving Fedora as well. Once you actually get Solus installed, the system is so stable and reliable I feel you wouldn't look back.
JR
Disks | Solus Help Center
UEFI
If you are using a system with UEFI, you may need to create a EFI System Partition, also referred to as an ESP. This is not necessary if you are enabling Solus to install onto the entire disk.
To create an EFI System Partition, open up GParted and create a FAT32 partition that is 512MB in size. Next, right-click on the partition and click Manage Flags. On the Manage Flags section, enable the boot and esp flags.
Notes:
Your system must be booted using Unified EFI (UEFI) mode, as opposed to a “legacy (BIOS) mode”.
Secure Boot needs to be disabled.
Your disk is required to be GPT formatted.
If you cannot see your SSD drive, set the SATA configuration to AHCI.
