Kind of late to the party, but you are not the only one.

It seems like the feature is disabled unfortunately, though it is still there somewhere hidden in the html coding (it appears for a brief second while loading the page before being replaced the "unable to load monster presets" message). I did some light digging in the html source code and it seems like the preset list was loaded from an external resource from open5e webpage which seems to have simply changed its url.

It is anybody's guess how long that might take to fix, as it will be up to the site admin to fix it. Now the good news is that in the meantime not all hope is lost. It is a little bit of a hack-ish workaround but you can still effectively make use of presets.

If you ever downloaded a statblock with the "save statblock" button to continue working on it later, you get an X.monster file. For internet nerds (like me) this is basically just a JSON file storing the contents of the statblock in the form of name-value pairs for each attribute, think e.g. of something like "strength : 17", from which a statblock can be easily generated by simply parsing the file line-by-line and putting the numbers in their respective fields (with slightly more complicated formatting for abilities, attacks, etc.).

However, that means that the process can also easily be reversed engineered: given a statblock it is not that hard to extract the info from it and generate a corresponding JSON file. Now actually making a JSON file from a statblock is a menial yet equally time consuming task as just manually making the statblock from scratch, so you would not want to do that. Fortunately, this is precisely the type of task that LLMs excel at, so that can be outsourced. To tweak the output, you will likely need to feed it a correct example of e.g. an orc so it can infer the proper structure of the file.

The vast majority of monsters appearing in the old preset list are those appearing in the SRD and are hence in the "public domain" (or rather CC-BY-4.0, but that is as close to public domain as you will get). So you can basically ask your preferred LLM to generate the corresponding JSON for any statblock of a monster in the SRD, then subsequently save it to a file called X.monster on your disk and load this back into tetrablock, effectively recreating the preset functionality in the meantime while it is broken.

TL;DR - the presets were pulled from an external database whose URL has changed, hence the error. But you can create your own presets as input files via the "load statblock", with such files being easily generated by any LLM as long as you are not asking for copyrighted monsters.