QGIS server + spawn-fcgi : where can I get informations when an error occurs?

Question

I am new to GIS, QGIS and spawn-fcgi. Thus, the answer to my question may be obvious.... but I couldn't figure it out (or express it in a way an accurate way such as I find accurate answers!)

Here it is : I retrieved a docker Image of QGIS server (3.0) that is served through a (also docker-hosted) nginx server and spawn-fcgi. The configuration looks very standard, and exactly follows QGIS Getting Started tuto.

The .qgs project provided as as example runs well : I perform some WMS requests on it, and retrieve the correct data.

But when I switch to one of my projects (whose WMS capabilities have also been set), I get an error. Whatever the request is, it keeps failing:

• http://172.....:8080/qgis?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&Layers=mylayer&BBOX=1,2,1.1,2.1&...
• http://172.....:8080/qgis?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetCapabilities
• http://172.....:8080/qgis?SERVICE=WMS
• http://172.....:8080/qgis?SERVICE=toto

=> 500

<ServerException>Project file error</ServerException>

I followed a few tips provided on this post , but though I fixed some volume mounting problem, I still have my pb.

The project is present under the expected directory (specified by QGIS_PROJECT_FILE):

$ qgis exec -it mycontainer bash
qgis@mycontainer $ ls ${QGIS_PROJECT_FILE}
/data/myproject.qgs
qgis@mycontainer $ 

Also, when I consult logs (with debug activated and log level set to all :'0'), I see the WMS requests :

[21][09:12:17] ******************** New request ***************
[21][09:12:17] REMOTE_ADDR: 172.18.0.1
[21][09:12:17] CONTENT_TYPE: 
[21][09:12:17] HTTP_USER_AGENT: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0
[21][09:12:17] REQUEST:getCapabilities
[21][09:12:17] SERVICE:1.3.0
[21][09:12:17] Request finished in 8 ms

The ngninx logs show the 500 error... and that's all.

And here's my point : how do you get informations about qgis server errors?

Looks like I should have a look to spawn-fcgi logs... but I am quite lost here : I eventually have no clue on where I should look for them, or whatever informations about how to understand (and fix!) my problem... So any tips are welcome!