Elna 3007 reinforces every stitch.

My guess is feed dog interference. Clean out bobbin, hook, and feed dog area.

You might first try lowering and raising feed dogs and see if problem goes away.

https://www.manualslib.com/products/Elna-3007-5542860.html

Elna 3007 Manuals

Start out with the tension set about midway. The tweak it. The two threads should meet in the middle of the fabric.

Thread Tension

How to Achieve Ideal Sewing Machine Thread Tension Threads

Sewing Lesson 10 How to Fix Tension on Your Sewing Machine

try this link for the manual
Elna 3003 3005 3007 Instruction Manual

try this link for a manual

Elna 3003 3005 3007 Instruction Manual

Have you found the blind stitch foot? On my Elna SU it has a piece of metal shaped a bit like a boat rudder in the middle, going front to back. Do you know which stitch does blind stitch? On my Elna it does two or three straight stitches and then one zig zag stitch. Make absolutely certain the needle is in the centre position before you start, if not, it will get expensive on needles! Fold up your hem and press it, and tack it into position. Blind stitch is a bit different, you need to position the presser foot so that the hem is on the right hand side of the paddle, and the "body" of the item is to the left. In order to do this, you need to fold the body of the fabric again (but don't press it or it will leave a line). I'd suggest a few practice runs first on scraps of fabric, so that you get the fabric positioned correctly. It will also take a bit of fine tuning to get the needle to just take a tiny bit of the body of the item. I found it a great stitch for things like curtain hems, and fairly heavy fabrics like tweed, because done properly it's far stronger than hand sewing and the stitch hides itself in the fabric, but I find it difficult to get it just right.

Check to see if your needle is bent.

Does your manual have a troble shooting section at the back and if so, what does it indicate for the E2 message?

Have you replaced the foot with the correct one and gone back to a straight stitch setting?

I'd turn it off, remove the needle plate and bobbin, clean everything out, check that there isn't a broken bit of needle anywhere, then reset everything up, turn it back on and try again with the standard foot.

Mechanical.

Here is an overview - http://www.oksewingandvacuum.com/3007.htm

50 year old elnas made stitches by cams. look to see if the machine has a cam in it. you can see this by opening lid on top of machine. cams are black with red or white showing the pattern of the cam. could also have broken cam stack gear. That would require technician with elna experience