Hello,
I reported that on the github of Iris the problem but apparently there is a compatibility problem between Iris and multiconnect. Here is the issue on the github of iris IrisShaders/Iris#585

Here is what I posted about the issue:

For me:

Screenshots

Minecraft 1.17.1:

Minecraft 1.17.1 with multiconnect on 1.13:

Minecraft 1.17.1 with multiconnect on 1.12.2:

Minecraft 1.17.1 with multiconnect on 1.8.x:

Relevant log output

iris-logs.txt

Iris and Minecraft Version

iris-mc1.17-1.1.0+rev.01d8631.jar for minecraft 1.17.1
multiconnect 1.4.6 because I was not up to date on the video but now I have tested with multiconnect 1.4.8 and still the same problem

Are you running Sodium along with Iris?

I tried with / without sodium (0.3.0-IRIS1-rev.faa155f), lithium, starlight, sodium-extra, hydrogen, falling leaves and EntityCulling.

Operating System

Operating System: Arch Linux
KDE Plasma Version: 5.22.3
KDE Frameworks Version: 5.84.0
Qt Version: 5.15.2
Kernel Version: 5.10.50-1-lts (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 4600H with Radeon Graphics @ 3.000GHz
Memory: 15.1 Gio of RAM
Graphics Processor: AMD Renoir (processor) and AMD Radeon RX 5500M(dedicated)

What is your GPU on minecraft ?

AMD Radeon RX 5500M

Java Version

openjdk version "16.0.1" 2021-04-20
OpenJDK Runtime Environment (build 16.0.1+9)
OpenJDK 64-Bit Server VM (build 16.0.1+9, mixed mode)

Additional context

To use it without problem load the shader on a multiconnect version of minecraft than 1.13 or on the main menu.
Example: https://youtu.be/OPKkRD5gjLQ