oh hey that's some useful advice. somehow I have missed that this is possible to disable, thanks

Added as "IsMarkedIS()" in today's update, enjoy!

Yes, I misread, I wasn't aware there was a mod called Item Saver. I will add in support in the next update, thanks for the request!

The presence of FCOItemSaver support is why he's written the additional support function for ItemSaver, which serves the same purpose as that mod. He acknowledged it when he said "(Which is different from FCO Item Saver)". Some people just choose to use it instead of FCOItemSaver because of familiarity or specifics of implementation. Personally, starting as a newbie in February, I looked at both and decided that FCOItemSaver was the better choice for me, but perhaps the choice is more nuanced for the next newbie if integration with ItemSaver is added to this mod.
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Hey, you may have missed that Auto Category already has FCO ItemSaver support via the IsMarked() function. Does yours do something different? I am not at my computer right now to take a closer look.

(Which is different from FCO Item Saver)

I've written a function and added ItemSaver to the OptionalDependsOn:
It is working for me thusfar, but I'd like it to be added in so that my friends can download and use the code too.

You have a marvelous addon! I'm growing quite fond of it.

This is much more helpful than simply stating that it crashes and offering nothing else. Perhaps there is indeed something unique about ownership of the guild that is being accessed that causes trouble.

Yes, I have similar crashes, and narrowed it down to this addon. It happens even if I temporarily turn it off with the keybinding before trying to open the guild bank.

One more thing I noticed, that it only crashes when I try to open my own guild's bank, it's not crashing if I try to open another guild's bank (where I have limited access, if it might have anything to do with it?)

Small update: I removed all rules from my guild bank settings, and that way it does not crash (yet).

I understand your approach now. I'll change the rule to include that and take the if statement out of the function, and see if the result is the same over time. It will have to be changed if they ever increase the maximum gear CP level, but that would be true of my if statement, too... and you'd rather not have me be asking you to change that when that happens.
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I still don't think you are understanding what I'm saying. The basic issue is that CP160 item should never be classified as low level, right? So what I'm saying is that you should just check at the beginning of your rule whether the item is 160, the rest of the code is irrelevant. Pseudo-code:

(itemcp != 160)
AND
(rest of your tests)

This will return false if the item is CP160, so it will never be marked as low level. Character CP is irrelevant in this case.

I'm not sure why you changed your original reply to this, but I can answer this one more directly. No, I don't have characters that can wear gear that has a CP value greater than 160, nor does that gear exist yet; you know this. The issue was and is that the charcp() function as I originally imagined it only returns the effective CP level of the character, a value that is worthless when trying to determine whether a gear item is above or below the "gear CP level" of a character. Here's a thought experiment:

I have three characters, one that is CP100 and another that is CP300. I have a piece of CP160 gear in the bank. You and I both can easily see that it will be "low level" to NEITHER of those characters. With my original Low Level rule,when each character visits the bank, how is that piece of gear categorized?

The answer is that it's categorized as Low Level for ONE of them, the CP300 character, which is clearly wrong. Why does that mis-categorization take place? It's because of this test:160 < 300

Do see the problem and why I had to add the extra if statement to the function? Any character whose CP level is greater than 160 must have its CP level as reported by the ESO API capped to a range of 0 to 160, else the above test returns an erroneous result.
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If the item is CP 160, how can it possibly be lower level than the current character? Do you have characters that can wear gear >CP160?

I think you've forgotten the reason why I asked for those functions and altered the rule in the first place: I want the Category to match gear that is a lower level than the current character. Testing against a CP value of 160 makes it no different than the original rule, which didn't need the extra function(s).

What I thought you were going to offer was an elegant means to eliminate if cp > 160 then cp = 160 end from the charcp() function itself? I'd rather not be maintaining my own private fork of AutoCategory just for one sentence of code. For my rule to work, either that must remain in the charcp() function or an efficient equivalent needs to be inserted into the matching rule. I don't know how to implement that.

I will try to look into this, but I am not the original author of Auto Category and may not be qualified to resolve this type of issue. Guild Banks work OK for me, but are definitely a little slower to open. If you're mainly having issues with Guild Banks, just remove all the rules from your Guild Bank settings. That will work around it for now.

Just add an explicit check for whether the item is 160 with the cp() function.