Canon 100-400 1st gen

My Canon 100-400 1st gen push/pull lens cannot tighten anymore ... are there any solutions ?

It's against forum rules to say "do a search".
However if you do search, use this:

In Forums = Canon EF and EF-S Lenses
Title = Friction

That results in 4 threads with lots of info.

I don't know if search links work, I guess I'm about to find out:
EDIT: link removed, it does not work!

My 100-400 had the opposite problem - zoom barrel seized...

I think Canon has discontinued support for these lenses so you'd need to either find a repair center that would take it on, or do the repair yourself.
If it's any help, the zoom barrel is part number CY1-2833-000 and it should come with the tightening mechanism ring as one unit.
I say "should" because there are some sellers that remove the "zoom band" and sell it seperately
The band on it's own is part YA2-3642-000 but would require fitting of millions of tiny ball bearings so if it was a DIY job, I'd strongly suggest you get a complete zoom barrel.

My lens I took apart & removed the zoom band altogether- not really difficult and you don't need to upset any of the lens adjustments to remove/replace.

check out: https://photography-on-the.net …showthread.php?​p=18697752
there's a link in there to a French DIY repair blog that contains some info on 100-400mm repair.

Thanks for the info.