I am experiencing the worst issue. I'm saying the "worst" because it looks like it can't be solved.

My situation is like this:
I was making a setup with vellum weld and vellum solver and the console gave me the weirdest error (screenshot attached). I've never seen it before.
Then I tested with an even simpler more reduced setup. The error appeared again.
Then I downloaded the official .hip file for the Vellum Nodes: https://www.sidefx.com/tutorials/vellum-nodes/ [www.sidefx.com] and again the error appeared.

I contacted support and after 7 e-mails of screenshots and back-and-forth experiments they just kindly said nothing can be done at the moment.

This is an issue because my Houdini license is through Steam. In my mind, it seemed easier to buy it through Steam, but now if I didn't have Houdini Indie through Steam, I could use the Daily Build 19.5.414 (where this Vellum Solver problem is fixed) instead of the official 19.5.403.

I don't know what to do. People from support told me there's nothing they can do since I cannot install the Daily Build because I don't have a license for it since Steam License can't be moved. They said they will release the 19.5.414 next month.

Now I work in Houdini every day, waiting a month is impossible for me.

If anyone has any ideas of what to do now, I would really appreciate it.
Personally, my advice is to not buy it from Steam since now I regret it.

As a temporary workaround, you can try the following:

1. Right-click on the Vellum Solver SOP and choose "Allow Editing of Contents".
   
2. Dive into the solver and look for the green "dopnet1" node.
   
3. Dive into that node and look for the "vellumsolver1" DOP.
   
4. On the Advanced tab of that node, look at the bottom for the "OpenCL" section.
   
5. Under the Kernel Options parameter add the following text: -cl-std=CL1.2

The other (easier) option is to turn Smoothing Iterations down to zero, but that will likely affect the quality of the simulation.

With that Kernel flag added, you might run into complaints from the compiler when using Pressure or Shape Match constraints, but you can likely silence those by turning off the faster OpenCL 2.0 path for them as described in the Note here:
https://www.sidefx.com/docs/houdini/news/18_5/vellum.html [www.sidefx.com]

Thanks johner for the workaround. I stopped tweaking Houdini at one point and I went for the GPU driver.
After 12h of e-mails me and the person from support finally cracked it.

I went to Nvidia and chose the 516 driver for Windows for 3080 Ti, here: https://www.nvidia.com/Download/driverResults.aspx/190348/en-us/ [www.nvidia.com]

And this just solved it like a miracle.

Since I have a pretty standard PC rig for Houdini, I don't understand how Vellum Solver would just stop working like that, since it's a very versatile tool.
I tested it with everything, grains, cloth, weld, etc.

Did this happen to anyone else?

Yes, it's an issue with the latest NVIDIA driver, which changed some things about the OpenCL compiler. This forum post is from people who ran into it:
https://www.sidefx.com/forum/topic/86887/ [www.sidefx.com]

We've fixed it for the daily builds, but there are ways to workaround if you're stuck on a production build.

Just curious if you guys think this is related.
Had issues with Vellum recently, where Houdini crashed when hitting play on Vellum sim, however I get no error message in console, it just crashes.
Almost instantly, no freezing, just crashes.

Edit: Okay, the mentioned Driver rollback indeed solved the issue.

I've been having the same issue, if I use the weld constraint in vellum Houdini just freezes and crashes. I've tried every driver I could and the OpenCL command line fix suggested by johner but still nothing works.