The belt failure you have described is characteristic of overheating due to slipping. Either you had insufficient tension or you overloaded the belt by taking on an impractical task. I can't tell which from here, and can't open the parts manual which is in the manuals section of this site - it seems to be corrupt. Hopefully someone else will advise us both on whether a 5-26 can be used for slashing.
If the drive chain is slack, the cause is likely to be wear on the chain and sprockets. Removing a link will accelerate the rate of damage to the sprockets, and the new chain will wear out quickly if the sprockets are already worn due to running with a worn chain in the past. You can assess this by inspecting the shape of the sprocket teeth. If they have long sharp points, or even undercutting of the points, the sprockets are very severely worn and will destroy any new chain you fit. It is correct practice to replace chain and sprockets as a set, in most cases.
