"Abstain from every form of evil" - 1 Thessalonians 5:22 - New King James Version
"Abstain from all appearance of evil" - 1 Thessalonians 5:22 - King James Version
The Greek word "pantos" can always be translated as either "all' or "every", depending on English grammar needs. In this case, it doesn't matter. There is only a difference if you add other words, like "once," to every or all, which 1 Thess 5:22 doesn't do. ("Every so often," or "every once in a while," both very different from "all the time," or "in every case," which mean the same thing.)
The word "eidous" is the key word, and can mean form, outward appearance, kind - basically, things you see.
The KJV misleads a bit with the English "appearance." The idea isn't to avoid people thinking you are doing evil, or to avoid doing what looks evil, but to avoid every kind of evil that you know/see. NKJV and NIV and ESV use "form" or "kind," instead, and rightly. The KJV leads us to think the meaning of eidous is outward only, going beyond the inward (as used in 1 Sam 16:7 - man looks at the outward appearance), when eidous actually means for us to avoid every kind (or all kinds) of real evil that we see. 1 Sam 16:7 and John 7:24 use a different Greek word (opsis) for the meaning of outward only, not (or going beyond) inward.
This idea of avoiding the appearance of evil gets abused, so that people are thinking more of what others think of them, than of avoiding actual evil. This can lead to declaring things off limits that actually aren't, and being more concerned with what others think instead of with what God thinks. It is good to avoid people thinking you are doing evil, so as not to stumble them (Rom 14:19-21; 1 Cor 8:9). But you don't have to swear off things that aren't actually sinful, either (Rom 14:22).
Any interpretation and even translation of Scripture is fallible. Holding to any one translation as the only right one is unhealthy, though some are better than others.