My Husqvarna started having problems with the bobbin. The thread from the bobbin side is a tangled mess when I stitch. The top looks fine, but the seam falls apart easily and the backside is all tangled. I cleaned underneath the bobbin holder but it didn't help.

is it the top thread that is too loose and making the messy stitch or the bobbin thread. if it is the top thread coming down and making large loops that tangle increase your top tension slowly try a few stitches, repeat till you get back to normal.
If it is the bobbin thread that is tangling, first make sure you are using the same thread top and bottom, some will just not work together, then rethread the bobbin, if it is still too loose there is a little screw on the side of the bobbin casing, the larger of the two, turn it no more than a 1/10 turn to the right if too loose and to the left if too tight. I would make the starting point with a piece of tape or nail polish so you can get back to where you were if this is not the problem. My book says you very rarely ever have to touch the bobbin tension. good luck

Thread nesting in bottom usually means that top thread is either not threaded correctly or top tension dials has lint caught in it. Use canned air to blow it out. Also change needle.

EIther the top of your machine is not threaded properly, or More than likely, your bobbin is not installed correctly.
If the attempt to sew is a mess on the back make sure your bobbin is spinning off the bobbin in the correct direction when installed into the bobbin case.

You may have a burr from an errant needle strike on your bobbin case or even on the bobbin itself. Check to see if there are any rough spots on or around the bobbin.
If you don't find anything, you may want to remove the bobbin assembly and clean the lint out from the lower assembly. Sometimes the smallest amount of lint or pieces of thread can cause problems with the bobbin.
You didn't mention if the tangled thread happens on the top or the bottom of the fabric. If it is on top, then it is a bobbin issue. If it is on the bottom, then it is an upper tension/thread issue.
Try removing the bobbin, thread, and needle. Rethread the entire machine using a different bobbin and a new needle. This may solve your problem. Good luck!

I'm guessing it is because the tension is not just right, the bobbin might not be the right size, the thread type top and Botton might not be the same, or you just might need put in a new needle the right size.

try changing the needle
try a different bobbin--make sure the bobbin is turned the right way
in the bobbin case
also try a different thread
make sure the top has not come off the thread take up lever
also the machine may have a timing problem and need to be
serviced

Be absolutely sure that the machine is threaded correctly, with the upper thread clicked into the tension discs (the presser foot MUST be raised when you thread so the tensions discs are open to receive the thread) and bobbin thread clicked into the bobbin tension slot. Change the needle - a bent or blunt needle can cause tangles. Hold onto the thread tails for two or three stitches when you first start sewing to avoid them being drawn down under the fabric and making a mess.

Clean any tangles from the bobbin area.

Always thread a sewing machine with the presser foot up so the thread can properly seat into the tension control.

Hand walk a few stitches to see if the thread still feeds too fast from the spool.

When thread pools (falls off the spool) a thread net will help - you can get these at a sewing machine dealer. I use to find them at fabric stores, but have not seen any lately with sewing/serging notions.

This could either be due to the bobbin being wound too loosely, or the tension on the top thread is too loose. This just happened to me the other day, and I thought it was the bobbin thread, but then I put another bobbin in with a different color than the top thread (this is a huge help in diagnosing thread issues), and I was surprised to find that it was actually the top thread that was a big loopy mess on the underside of the fabric. I fixed it by tightening the tension on the top thread (quite a bit), and everything was fine.

To address my needle thread tangling (horribly) in the bobbin compartment, I rethreaded the bobbin. I cleaned the bobbin and the feeder feet, I changed the bobbin and the thread. I changed the tension on the needle thread and the bobbin thread. NO FIX! Then, I changed the needle - the problem was fixed instantly. I sewed awhile and then put the original needle back in. No problem. I think perhaps that the problem is being caused by the needle getting out of alignment somehow during sewing. If you are having the same issue, try it. It can't hurt!