Repeated measures ANOVA for an experiment with missing values (unbalaced)

#1
I have an experiment where several subjects (subjects={S1,S2,..Sm})were asked to perform a set of tasks (tasks={T1, T2, T3, ..Tn}) using both their left (L) and right (R) arms. Each task for each arm was repeated 'r' times. The response is measured in a variable called 'measure'. Unfortunately, there were some tasks and repetitions missing.

I tried using the aov function in R to perform a repeated measures ANOVA analysis, but later found out that this is not appropriate for unbalanced designs. Here is the sample of what I did:

> summary(aov(measure ~ arm*task + Error(subject/(arm*task)), data=all_data))

Error: subject
Df Sum Sq Mean Sq F value Pr(>F)
arm 1 0.3240 0.3240 0.398 0.573
task 4 0.1426 0.0357 0.044 0.994
Residuals 3 2.4397 0.8132

Error: subject:arm
Df Sum Sq Mean Sq F value Pr(>F)
arm 1 0.0023 0.00234 0.074 0.8027
task 4 0.9972 0.24931 7.941 0.0601 .
arm:task 1 0.0112 0.01117 0.356 0.5928
Residuals 3 0.0942 0.03139
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Error: subject:task
Df Sum Sq Mean Sq F value Pr(>F)
task 5 0.3898 0.07795 1.652 0.172
arm:task 4 0.1482 0.03706 0.785 0.542
Residuals 35 1.6511 0.04718

Error: subject:arm:task
Df Sum Sq Mean Sq F value Pr(>F)
arm:task 5 0.0352 0.007032 0.351 0.878
Residuals 35 0.7013 0.020036

Error: Within
Df Sum Sq Mean Sq F value Pr(>F)
Residuals 203 1.288 0.006345
Warning message:
In aov(measure ~ arm * task + Error(subject/(arm * task)), :
Error() model is singular

Why am I getting the warning message: "Error() model is singular"?

I also found that for unbalanced design it better to use the function anova() from the "car" package. I tried to search for the documentation for anova(), but I am very confused about how to use this function. Which model must I use for testing the following hypothesis:
1. Within-subjects there is no difference between the left and right arms.
2. Within-subjects there is no difference between the different tasks.
3. There is no interaction effect between arm and task within a subject.

It would also be very helpful if someone could point me to suitable books that can help me learn about these concepts.