It occurs due to protein denaturation and aggregation (clumping) from heat. To fix this, make sure to pre-chill equipment. Sonication produces intrinsic heat, so be weary. You may also want to increase the volume of lysis buffer, however, this is at the consequence of a lower protein concentration.

The solution I recommend is to pass your samples through a 27-gauge needle until the solution becomes less viscous. Bubbles or foam may form, but centrifuging helps.

Hi Zunair and Stjepan

The recipe I used was 10 mM Tris pH 7.5, 100 mM NaCl, 2 mM EDTA and 0.5% deoxycholate. There was not other detergent.

The buffer forms a gel-like itself so I wasn't even able to pipet it into my samples. On vigouros shaking the gel 'dissolved' to become viscous liquid. Is this normal for deoxycholate-containing buffers like RIPA?

The culprit is deoxycholate

Article Deoxycholate-hydrogels: Novel drug carrier systems for topical use

Good Answer Esteban Blanco Camejo