SUMIF statement with two different conditions (Text based and checkbox)

Just to make an official answer:

Use the SUMIFS() function to use 2 or more conditions.

From the documentation:

The SUMIFS function returns the sum of the cells in a collection where the test values meet the given conditions.

SUMIFS(sum-values, test-values, condition, test-values…, condition…)

As pointed out in the comments, I missed the SUMIFS in the functions list because I was focusing on SUMIF.

I am now using SUMIFS and simply attaching all criteria like in a regular SUMIF function and it works like a charm.

Example: SUMIFS(Hours;Billable;true;Finished;true)

Meaning: Summarize all hours If BillableHours-Checkbox is TRUE and if WorkFinished-Checkbox is TRUE.