Apologies, slightly long post.

Recently had a new bathroom as part of new extension, floor is green
backed chipboard, which was then tiled over. A week ago I was in there
and heard a loud crack, 1 tile had cracked, over the course of the next
couple of days 3 others the same. Tiles were put down with flexible
adhesive.

I have today pulled the cracked tiles up and the cause looks like it is
because the wooden boards underneath have expanded from each other
leaving gaps, which is where the cracks are.

Gaps look like they were there when the original tiles went down ( as
there is adhesive in them) but may have expanded (posibly due to
building settlement). Gap between 1 set of 2 boards is approx 4-5 mm
and between second set approx 2-3 mm. 2-3mm deep

first question, is there anything I can fill these gaps with to provide
a solid base to prevent the same problem again?

second Q, whilst I was pulling up the tiles, the set adhesive also
ripped up parts of the green backing on the board exposing the
chipboard, is there anything I can/need to do to make it safer ior will
the ahesive deal with that.

Any help appreciated for a novice tiler

Thanks
Gaffar