The Problem:
If I try to boot the OS I get a "corrupted page table" error, even though I can access my system/files just fine using the GUI rescue mode of the Debian online Installer (the original medium I used to install the OS).
edit: even though I don't think I changed anything, I now instead get a different Error: "Volume group "[groupname]" not found"
"Cannot process volume group [groupname]". I'm not sure if that's better or worse.

How it happened:
I tried to upgrade from stable to testing. During the upgrade process I somehow managed to accidentally remove many packages, which rendered me unable to connect to the internet.
First I tried to somehow reinstall the packages despite not having internet, which is apparently possible, but I couldn't figure it out. At this point at least booting my system was still possible.
Then I gave up, made a backup of my home folder and proceeded to boot the Debian Installer in order to reinstall the OS. But there I found the "rescue mode" and tried it. Sure enough was able to get an internet connection and an "interactive shell" on my system. I used this shell to reinstall (hopefully) everything I deleted. I used the standard apt commands to do this.
But after this I now get the "corrupted page table" error at boot (after grub).

More info:
Could I just reinstall the OS: Yes, but I'm rather new to GNU/Linux and would like to learn to how actually to fix stuff (or preferably not break it in the first place).
My system is installed on a single 2TB HDD with only 500GB allocated, because for some reason the installer recommended it. I chose to use the ext4 format and the LVM with the standard fulldisk encryption, iirc truecrypt.
The installer rescue mode gave me different options on where to open the shell, I always used the preselected /dev/debianPC-vg/root option.
Using lsblk in the installer rescue shell I can see 3 partitions on my drive:
sdc1 (487M, part, mounted in /boot)
sdc2 (1K, part, not mounted)
sdc5(1.8T, part, not mounted)
..|-sdc5_crypt (1.8T, crypt, not mounted)
....|-debianPC--vg-root (464.7G, lvm, mounted in /)
....|-debainPC--bg-swap_1 (976M, lvm, not mounted)
Using blkid in in the initramfs shell I get dropped into after failing normal boot, I only get 2 partitions (this is after getting the new error, I didn't try before):
/dev/sdb1 type=ext2, blocksize=1024, UUID, PARTUUID
/dev/sdb5 type=crypto_LUKS, UUID, PARTUUID
Back in the installer shell, I used the analyze function of testdisk on /dev/sdc, which finds all 3 partitions and displays "Structure: Ok" for all of them. Testdisk labels them as "ext2", "Linux_LUKS" and "CramFS".

Any help will be greatly appreciated, but please keep in mind that I'm still somewhat new to Linux.