Furthermore, a major release is very likely to break mods, and in this case, there absolutely are crashes when trying to use VS1.14 mods with the 1.15 pre-releases.
Some mods have already been updated for 1.15 - that was, as I understand, a large part of the reason for pre-releasing to begin with. But if it doesn't say it's been updated for the new version, it's safest to assume it isn't. If you aren't feeling quite patient enough to follow redram's advice, you can start by disabling all mods that aren't explicitly for the new version, and begin troubleshooting from there - but if even then it still crashes, you'll have to start from vanilla.