Ah, this particular issue with steam. If it is not a client-server asset mismatch EOF error, which can happen sometimes; in-which case you wait for the server to restart or avoid the areas the EOF occurs in; which you use F + W if you have distortionsphere equipped and "Aurora's SB Mod 1.0" on steam installed, in order to quickly move away from that area.
If however, as stated earlier; it is not a client-server asset mismatch EOF error. Then is the more complicated method. Workshop isn't easy to reinstall the files specifically for Starbound.
Note: Make sure to backup your player files before you begin. Player files are found in \Steam\steamapps\common\Starbound\storage\player
If you're not sure where your steam is, then right click Starbound and click properties then go to local files then click browse local files. Then you can move out of the folder structure until you reach the main starbound folder, which can be done using backspace or clicking at the top bar to go between folders.
- Go to the Starbound workshop
- Go to Your Workshop files area and hover over Your Files
- Click subscribed items
- Click Workshop items at the top.
- At the top of the screen click Filter by game > press select a game and type in Starbound.
- Click on Starbound.
- Scroll down to Items, Favorited, subscribed items and played section; make sure subscribed items is selected, if not then select it.
- Manually unsubscribe each addon. Make sure to right click the page and press reload when you're finished, just in-case some addons get missed.
- Close Steam
- Go to \Steam\steamapps\workshop\
- Remove appworkshop_211820.acf
- Go to \Steam\steamapps\workshop\content\211820
- Make sure no folder is present inside, and if there are any. Ctrl + A then Shift + Delete, to delete them all.
- Startup steam again.
- Subscribe to each addon manually in the workshop collection. Note: Do not click subscribe all! <- This will not work if you do; as you'll have to unsubscribe to all again then subscribe manually all over again!
Then you right click the game in steam click properties > local files > verify integrity of game cache.
Assuming you've done everything right, you'll just need to open the game, then restart the game; then wait 5 - 10 minutes, or how ever long the mods took to install for you the first time; then restart the game again. If all is well, this should load the mods in again, keeping your characters in-tact; and allowing you to join again.
If problems still persist, reinstall the game after doing all of this, which is another reason why you made or hopefully made those player file backups earlier. Note: if you do reinstall the game you must go to options in-game and tick allow assets mismatch! And mods do not apply until you've restarted the game typically when reinstalling the game; but it's always good to do that just in-case.