Users of Advanced Shipper:

When doing your upgrade from 1.5.4 or lower to 1.5.5 or higher, you might get stuck in zc_install with the log

[11-Feb-2019 09:23:52 America/New_York] MySQL error 1406 encountered during zc_install:
Data too long for column 'shipping_method' at row 12270
ALTER TABLE orders MODIFY shipping_method VARCHAR(255) NOT NULL DEFAULT '';

Your symptom will be the message in zc_install which will be something like:
"Cannot upgrade to 1.5.6; your database must be updated to 1.5.5 first."

Your best bet is probably to remove the check in includes/systemChecks.yml (line 299-303).

This issue was originally reported here:
https://www.zen-cart.com/showthread....1-5-3-to-1-5-6

I'm just adding this thread to help people searching for this problem.