I started with a clean, empty database initialized with the scripts in 2.5.1 (it also failed in 2.5.0 release, and also in an updated 2.4.6rc1 database).

It does not fail for every template: I have not been able to find a pattern for when it does and does not. In fact I have been able to full clone a failing template and have it not fail in the clone.

To reproduce: drill down into the Template OS Linux, discovery and into trigger prototypes for "Free disk space is less than 20%".

Take the initial value, and add an "and" condition for free space (not percent) as follows:

{Template OS Linux:vfs.fs.size[{#FSNAME},pfree].last(0)}< 20
and
{Template OS Linux:vfs.fs.size[{#FSNAME},free].last(0)}<200G

When you try to save it fails with a somewhat nebulous error (in fact it first says updated, then it shows a failure). See first screen shot.

If instead you use the expression constructor, it looks happy (but will not update), and if you go into the Test, you get what is perhaps an unrelated undefined property error (second screen shot).

Notice in the third screen shot I did a full clone, and did the exact same edit (+/- the cloned name). This was accepted without error (see fourth showing it in place). Note that the Undefined Property error will still appear in the Test screen for this working case, making me think that may be an independent error.