Out of curiosity, I got up and went through my pack to see if I could find the old character. Now that I have it in front of me, I can more accurately put some numbers to the scenario. My save DCs with the pregen started at 11/12 (depending on if I had a focus.) So, Cybit is right in saying the DCs were 13; after leveling up the prefen, my numbers were 12/13.
The actual word for word description of Web is essentially what I said above.
I did make a mistake with Web not benefiting from the evocation ability. However, I do not feel that would have significantly altered things. Most of my allies were focused on ranged attacks with a high dex and no reason to move through the webs anyway.
As for whether the DM interpreted the spell wrong? Possibly, but his interpretation (which even I as the player of the wizard felt was far too good) doesn't seem to contradict how the spell's effect is expressed. Perhaps the wording of the spell could be made clearer if that is not the intent.
As I sit here now, this is the first time I've actually read the Murder In Balder's Gate adventure, so I'm taking educated guesses at what the creatures faced in the final battle were. We were fighting in a lighthouse; there was a person empowered by an evil god (Bale maybe?) and some soldiers under his command. Between the thug, tough thug, watch soldier, and watch sergeant, the best Dex bonus is a +1; +0 is common. The best Str bonus is +2, which is better, but using that option of getting out of web requires taking an action, and then still again being on the webs at the start of your next turn and getting stuck again. I don't remember who the person in charge of the enemies was, but I'm going to say it was likely one of the people with the higher Dex bonus, so, in that case, I can only assume the DM was rolling poorly. The DM is usually at the opposite end of the table from where I sit, so I cannot always clearly see what he's rolling.