"Incompatible mod" errors are one of the most common reasons a modded server refuses to start, and the good news is that the cause is almost always one of a small handful of things, not a mystery.
Read the actual crash log first
Before changing anything, open logs/latest.log (or the crash report in the crash-reports folder). Forge, NeoForge, and Fabric are all reasonably explicit about which mod caused the failure and why, guessing before reading this is the most common way to waste time fixing the wrong thing.
Cause 1: Wrong Minecraft version
The single most common cause. A mod built for 1.20.1 will not run on a 1.21.4 server even if everything else matches. Check the exact version listed on the mod's Modrinth or CurseForge page against your server's actual version, a version checker can confirm exactly what your server reports if you are unsure.
Cause 2: Wrong mod loader
A Forge-built jar will not load on Fabric or NeoForge, and vice versa, regardless of Minecraft version. This happens most often when downloading from a search results page without checking which loader tab you were on.
Cause 3: Two mods genuinely conflicting
Less common than the two above, but real: two mods that modify the same game system in incompatible ways (certain mixin-based mods, or two mods both trying to override the same vanilla behavior). The crash log typically names both mods when this happens. The fix is usually checking each mod's page for known conflicts, or trying one without the other to isolate which is at fault.
Cause 4: An outdated mod on a newer Minecraft version
A mod that has not been updated in a while may simply not support your current Minecraft release yet, even though it worked on an older one. Check the mod's most recent file date against your Minecraft version's release date as a sanity check.
A systematic way to isolate the culprit
If the log is not clear enough on its own, remove mods in batches (or one at a time on a small pack) and restart between each, backing up first. The moment the server starts cleanly, the mod you just removed is the one to investigate further.
Preventing this going forward
Keeping every mod's Minecraft version and loader consistent, and checking a mod's dependency list before adding it, prevents the large majority of these crashes before they happen. A server requirements tool is also worth checking if you suspect the issue is hardware rather than software. Tick Hosting's panel surfaces startup errors directly in the console view, no digging through log files over SSH required, on every plan including the free tier. Get started on Tick Hosting's plans page.