Hey, so I have been having the exact same issue with about 2 of the latest modpacks from FTB (FTB Univeristy 1.16/OceanBlock).
Thankfully, I have found a fix for FTB University so hopefully, this will work for you as well. I will try and test this with OceanBlock as well as it's something I really want to try out as well.
It looks as though the issue is with the way FTB downloads/unpacks the files into the vanilla launcher. I have scanned the logs and I can't seem to narrow down the exact issue so something is not quite right. Here's what I did next.
1. Downloaded the files via the FTB app like normal.
2. Clicked Play
3. In the vanilla launcher, go to Installations > Click on the folder icon next to the installation (in your case, OceanBlock) to open the folder
4. Run installer.jar and let minecraft forge unpack itself (close down the installer afterwards).
5. Copy the entire folder to somewhere else on your machine
6. Open the vanilla Minecraft Launcher program (no FTB etc)
7. You should see a version of forge installed from the previous step (note, it will not be called OceanBlock or have the same logo)
8. Click play and make sure it launches, you should have 2 mods installed (MC and Forge only)
9. Open the vanilla launcher again and go to Installations > Click on the folder icon next to the forge installation
10. Copy and paste everything from your modded MC (OceanBlock) into this directory, if Windows tells you that there are files with the same name, KEEP the original files in there.
11. Go back to the vanilla launcher Installations > Forge > . . . > Edit > More Options > Change -Xmx2G to -Xmx5G (5GB RAM) as recommended, 2G is not enough for this modpack
12. Click Save
13. Click Play and everything should load up fine
Obviously when there are updates it might be a bit more tricky to update everything but so far I've managed to load a new world just fine. I'll try and load up OceanBlock to see if fixes this as well.
Good luck!
