The new version of Steam has some new certificate bundled in, so you have to modify a file and add some code for Steam to install games.

1. Navigate to your game's wineprefix: /home/username/.PlayOnLinux/wineprefix/gamename/drive_c/Program Files (x86)/Steam/config/
2. Open config.vdf with a text editor
3. Copy this code below
   
   Code:

   "CS"        "valve511.steamcontent.com;valve530.steamcontent.com;valve548.steamcontent.com;valve539.steamcontent.com;valve519.steamcontent.com;valve555.steamcontent.com;valve504.steamcontent.com;valve529.steamcontent.com;valve558.steamcontent.com;valve500.steamcontent.com;valve561.steamcontent.com;valve532.steamcontent.com;valve540.steamcontent.com;valve502.steamcontent.com;valve544.steamcontent.com;valve528.steamcontent.com;valve556.steamcontent.com;valve520.steamcontent.com;valve542.steamcontent.com;valve538.steamcontent.com;valve551.steamcontent.com;valve521.steamcontent.com;valve564.steamcontent.com;valve516.steamcontent.com;valve535.steamcontent.com;valve541.steamcontent.com;valve533.steamcontent.com;valve536.steamcontent.com;valve557.steamcontent.com;valve559.steamcontent.com;valve537.steamcontent.com;valve508.steamcontent.com;valve525.steamcontent.com;valve565.steamcontent.com;valve566.steamcontent.com;valve514.steamcontent.com;valve545.steamcontent.com;valve517.steamcontent.com;valve547.steamcontent.com;valve549.steamcontent.com;valve503.steamcontent.com;valve515.steamcontent.com;valve506.steamcontent.com;valve562.steamcontent.com;valve526.steamcontent.com;valve543.steamcontent.com;valve522.steamcontent.com;valve518.steamcontent.com;valve509.steamcontent.com;valve550.steamcontent.com;valve552.steamcontent.com;valve505.steamcontent.com;valve553.steamcontent.com;valve512.steamcontent.com;valve546.steamcontent.com;valve554.steamcontent.com;valve510.steamcontent.com;valve563.steamcontent.com;valve501.steamcontent.com;valve531.steamcontent.com;valve523.steamcontent.com;valve524.steamcontent.com;valve507.steamcontent.com;valve560.steamcontent.com;valve567.steamcontent.com;valve513.steamcontent.com;valve1601.steamcontent.com;valve164.steamcontent.com;valve1615.steamcontent.com;valve1602.steamcontent.com;valve1604.steamcontent.com;valve1608.steamcontent.com;valve1603.steamcontent.com;valve1606.steamcontent.com;valve1609.steamcontent.com;valve1618.steamcontent.com;valve1617.steamcontent.com;valve1613.steamcontent.com;valve1607.steamcontent.com;valve1611.steamcontent.com;valve1614.steamcontent.com;valve1619.steamcontent.com;valve1616.steamcontent.com;valve1612.steamcontent.com;valve1610.steamcontent.com;valve165.steamcontent.com;valve1605.steamcontent.com;valve954.steamcontent.com;valve955.steamcontent.com;valve904.steamcontent.com;valve901.steamcontent.com;valve957.steamcontent.com;valve953.steamcontent.com;valve956.steamcontent.com;valve950.steamcontent.com;valve905.steamcontent.com;valve952.steamcontent.com;valve902.steamcontent.com;valve907.steamcontent.com;valve958.steamcontent.com"

4. Go to the line
   "InstallConfigStore"
   {
   "Software"
   {
   "Valve"
   {
   "Steam"
   {
   "CM"
   
   
5. Paste in a new line under the "CM" line with all the IP Addresses
6. Save config.vdf
7. Restart Steam