Okay, because I had to mess around quite a bit at first, I will make a quick how-to here:

-To upload to the workshop, you don't need to pack your mod, the uploader will handle it
-create a folder in a place that you best keep for quite some time (maybe make a mymods folder in starbound) and add a subfolder with your mods name (actually doesn't matter which name here, this is just for you)
-inside this subfolder, put the your mod. This may be your object folder, your recipes and so on, just like in the standard assets folder
-create a txt file in the subfolder and fill into it your metadata (or change your old modinfo file)in the form of-save the txt file, rename it to "_metadata_" or ".metadata_". The _ at the end here is for windows user, it enables you to name a file with only an ending.
-Now, go to your steam library and either rightclick Starbound or start it and choose the "Mod Uploader Tool". Alternatively you can also directly start the exe in the win32 folder in your Starbound folder.
-This now opens the mod uploader (duh). Choose "Select Mod Directory" and browse the subfolder you created (in our case it would be in Starbound/mymods/"yourmodname/". If your metadata is renamed and structured correctly it should read the information from it.
-You can now give your mod a name (aka the name it will be shown as in the game itself), a title (the name which is shown while browsing the workshop), a description, version number and even a preview image (definitely upload one, it helps tremendously gaining traffic)
-Anyways, after you have finished the details, click on "Upload to Steam!" and the tool should pack your mod together and load it onto the steam databases. If you have a mistake in your metadata file, you will get an error message already here.
-A number will appear under the "Mod ID", click it and it will port you to your now freshly created modsite where you can change a few things or also add some pictures or youtube links to it.
-When you first upload a mod, it will automatically set it's visibility to "hidden". That other users will be able to see that, change it to "public". Best is though that you try it out yourself before you do this, to ensure it is working properly. For that click on the green subscribe button and it should download your modfiles. Don't forget to remove any other files of your mod from the /mod folder, otherwise Starbound will load those.
-Congratz, you have successfully released a mod onto the workshop!

Updating a mod in the Workshop:
-To update your mod, simply update your files and choose your folder again. It should reload all the old information. Don't overwrite the metadata file though, as when you uploaded your mod the first time, the uploader added an additional line indication your mods ID.
-In case you deleted the folder already, repeat the steps above but add the line
to the metadata file. This line got added the first time you uploaded your mod and is the link between your folder and your mods workshop page. In case you missed it, you can find the id on your mods workshop page.
-If you also deleted the _previewimage file (stop deleting things, seriously!), which was just an encoded version of the preview picture you have uploaded, simply use the mod uploader to choose a new one and it should be generated again

for issues with steam, be sure of the following:
-You need to be online in steam and you need to not be in the locked family mode
-Be sure that you actually own Starbound
-Make sure that you are not logged into your account twice, e.g. you will get an error message if you are logged into steam in your browser as well as in the app!