I was upgrading an EE install from 2.5.2 to 2.5.5. First of all, I updated all the add-ons to make sure they were the latest versions. Then I added the new EE files as per the updating docs.
When running the upgrade, I got the following error message in a popup window after about 15 seconds:
The update script failed without returning an error. Please contact tech-support.
My hosting has plenty of memory (512mb) and execution time (60 seconds, though I even upgraded it to 600 seconds to rule it out as an issue) and has run dozens of upgrades before without any issues. I even tried upgrading from 2.5.2 to 2.5.3 instead, but got the same result.