The problem is that the ladders are accidentally disabled instead of being enabled, it's very similar to the common mistake with teleports which start on by default, and then are inadvertently switched off when called for.

I remember playing this map ages ago, I have a feeling it was like this even back then, so it may be a fundamental map error rather than a map-breaking game update. It could be either, really.

I have uploaded a fixed Ripent version, baseunderfire_fixed.zip. I've tested it and it doesn't cause "map differs" errors so you can safely upload it to your server without having to rename the bsp file.

You can complete the map solo if you choose - stand in between the base pillars of each tower, button on either side of you and if you turn quickly enough, you can press both buttons "together" to explode the crates and activate the ladder. If you can take the damage, you can jump off of the sentry onto the bottom of the ladder without any other cheats (if playing with others, you can easily boost off of someone onto the ladder). Once the bomb is planted at the top, return to the roof of the central building and press the appropriately numbered button to detonate the bomb.