when compiling my project I get errors related to the l3experimental package.

From the log file:

("C:\Program Files\MiKTeX 2.9\tex\latex\l3experimental\l3str\l3tl-build.sty"
Package: l3tl-build 2015/07/28 v5700 L3 Experimental token list construction

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!
! Control sequence \l__tl_build_start_index_int already defined.
!
! See the LaTeX3 documentation for further information.
!
! For immediate help type H <return>.
!...............................................

l.65 \int_new:N \l__tl_build_start_index_int

|'''''''''''''''''''''''''''''''''''''''''''''''
| This is a coding error.
|
| LaTeX has been asked to create a new control sequence
| '\l__tl_build_start_index_int' but this name has already been used
| elsewhere.
|
| The current meaning is:
|   \count126
|...............................................

\l__tl_build_start_index_int=\count305

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!
! Control sequence \l__tl_build_index_int already defined.
!
! See the LaTeX3 documentation for further information.
!
! For immediate help type H <return>.
!...............................................

l.66 \int_new:N \l__tl_build_index_int

|'''''''''''''''''''''''''''''''''''''''''''''''
| This is a coding error.
|
| LaTeX has been asked to create a new control sequence
| '\l__tl_build_index_int' but this name has already been used elsewhere.
|
| The current meaning is:
|   \count127
|...............................................

\l__tl_build_index_int=\count306


and the same with a lot of other control sequences. I am working on a min example where the error occurs. Thanks!

• Welcome to TeX.SX! Please help us help you and add a minimal working example (MWE) that illustrates your problem. Reproducing the problem and finding out what the issue is will be much easier when we see compilable code, starting with \documentclass{...} and ending with \end{document}. – ebosi Jul 13 '17 at 9:44
• I can't check with my install right now, but l3tl-build 2015/07/28 was a bit older than what I could find on the web. So maybe an update can help (make sure to update all packages!). If that does not help, we probably need to see an MWE. – moewe Jul 13 '17 at 10:14
• Mhhh, it seems the package was removed from l3experimental. Not sure if it (or its functionality) moved elsewhere. I guess your best bet is a full update of all packages involved. If the functionality has moved then other packages should call the new commands, if things were removed they should refrain from calling anything. – moewe Jul 14 '17 at 8:27
• The l3tl-build package was moved to the kernel. Normally there is a transition period where the package just produces a warning, but here we made a mistake in the markup which means the file l3tl-build.sty is not generated at all. Your installation has a stray file l3tl-build.sty. A very short-term solution is to put an empty file l3tl-build.sty in the directory of your TeX file. – Bruno Le Floch Jul 15 '17 at 23:08
• Fixed in the latest sources on Github. github.com/latex3/latex3 Should make it to CTAN in a few days. – Bruno Le Floch Jul 16 '17 at 3:49

The l3tl-build package was moved to the kernel. Normally there is a transition period where the package just produces a warning, but we had not done that for this case (somewhat by mistake). There is a good reason why l3tl-build was removed with no transition: it provides no user function!
Anyways, it's always better to have clearer error messages so now we generate a transitional l3tl-build.sty that produces an error.