In T8780#166820, @DataDrake wrote:

That's probably a red-herring. Not everything printed to the kernel log that looks like an error, is actually a fatal one.

Can you try booting without "quiet" and "splash" set so that you can see what other errors are printed?

Solus isn't installed yet, so how do I boot without quiet and splash?
Do I need to modify the GRUB file inside the Solus ISO?
Meanwhile, I've updated the BIOS (I had to install Windows 10 just to do that, because Acer only provides BIOS updates through Windows) and then tried again to install Solus.
This time, the IOMMU error is still there, but is followed by this:
"a start job is running for rebuild dynamic linker cache"
The start job keeps running forever, as it has no time limit.
I've Googled this error and found out that a lot of people are having this issue with the same laptop, so it's probably Acer's fault.
BTW, Secure Boot is disabled, otherwise it wouldn't even let me get to this point.